Description of problem:

The help of "openstack server create" says that --config-drive option accepts a volume.
~~~
(overcloud) [stack@undercloud-0 ~]$ openstack server create -h
usage: openstack server create [-h] [-f {json,shell,table,value,yaml}]
 :
  --config-drive <config-drive-volume>|True
                        Use specified volume as the config drive, or 'True' to
                        use an ephemeral drive
~~~

However, it fails when a non-boolean value is specified like this.
~~~
(overcloud) [stack@undercloud-0 ~]$ openstack server create --config-drive testvol1 --flavor m1.nano --image cirros-0.4.0-x86_64-disk.img "testvm1"
Invalid input for field/attribute config_drive. Value: testvol1. 'testvol1' is not one of [True, 'True', 'TRUE', 'true', '1', 'ON', 'On', 'on', 'YES', 'Yes', 'yes', False, 'False', 'FALSE', 'false', '0', 'OFF', 'Off', 'off', 'NO', 'No', 'no'] (HTTP 400) (Request-ID: req-UUID)
~~~

Version-Release number of selected component (if applicable):
* RHOSP13
* RHOSP16

How reproducible:

Steps to Reproduce:
1. Specify a non-boolean value to the --config-drive option for `openstack server create` command

Actual results:
An error occurs.

Expected results:
The help message should be updated like this because it's not implemented in nova-api:
~~~
  --config-drive True
                        'True' to use an ephemeral drive
~~~

Additional info:
According to the nova-api doc, it only accepts a boolean value.

https://docs.openstack.org/api-ref/compute/?expanded=create-server-detail#id11
~~~
config_drive (Optional) body boolean
Indicates whether a config drive enables metadata injection. The config_drive setting provides information about a drive that the instance can mount at boot time. The instance reads files from the drive to get information that is normally available through the metadata service. This metadata is different from the user data. Not all cloud providers enable the config_drive. Read more in the OpenStack End User Guide.
~~~