To be honest, I can no longer trigger the bug on unpatched master, but the idea was the following: you need to enroll nodes, then update them via instackenv.json by setting capabilities to a dict.

E.g. pick one node, and add/replace its "capabilities" in instackenv.json with "capabilities": {"profile": "control", "boot_option": "local", "foo": "bar"}. Then update the nodes via 'openstack overcloud node import instackenv.json'. The capabilities should be correctly converted to a string:

$ openstack baremetal node show control-0 -c properties                                                                                                                           
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Field      | Value                                                                                                                                              |
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| properties | {u'memory_mb': u'10240', u'cpu_arch': u'x86_64', u'local_gb': u'50', u'cpus': u'2', u'capabilities': u'profile:control,foo:bar,boot_option:local'} |
+------------+----------------------------------------------------------------------------------------------------------------------------------------------------+

13   -p 2018-12-13.4

Env:
(undercloud) [stack@undercloud-0 ~]$ sudo rpm -qa | grep openstack-tripleo-common
openstack-tripleo-common-containers-8.6.6-8.el7ost.noarch
openstack-tripleo-common-8.6.6-8.el7ost.noarch

original
controller-1 original capabilities
(undercloud) [stack@undercloud-0 ~]$ openstack baremetal node show controller-1 -c properties
+------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Field      | Value                                                                                                                                                                                                       |
+------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| properties | {u'memory_mb': u'20480', u'cpu_arch': u'x86_64', u'local_gb': u'39', u'cpus': u'4', u'capabilities': u'cpu_aes:true,cpu_hugepages:true,boot_option:local,cpu_vt:true,cpu_hugepages_1g:true,boot_mode:bios'} |
+------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------

Note: changed the boot_mode to uefi

.... instackenv.json

      "name": "controller-1",
      "mac": ["52:54:00:d4:82:b3"],
      "cpu": "4",
      "memory": "20480",
      "capabilities": {"profile": "controller", "boot_option": "local", "cpu_vt": "true", "cpu_hugepages_1g": "true", "boot_mode": "uefi"},
      "disk": "40",
            "arch": "x86_64",
              "pm_addr": "172.16.0.1",
        "pm_type": "ipmi",
        "pm_user": "admin",
        "pm_password": "password",
        "pm_port": "6231"

.....


openstack overcloud node import instackenv.json

You can see we successfully updated and did not have any error

(undercloud) [stack@undercloud-0 ~]$ openstack baremetal node show controller-1 -c properties
+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Field      | Value                                                                                                                                                                                          |
+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| properties | {u'memory_mb': u'20480', u'cpu_arch': u'x86_64', u'local_gb': u'40', u'cpus': u'4', u'capabilities': u'boot_mode:uefi,boot_option:local,cpu_hugepages_1g:true,cpu_vt:true,profile:controller'} |
+------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:0068