these 3 commands should work for orphaned network with subnets:
---------------------------------------------------------------

CloudNetwork.where(:type => "ManageIQ::Providers::Openstack::CloudManager::CloudNetwork", :external_facing => false).update_all(:type => "ManageIQ::Providers::Openstack::NetworkManager::CloudNetwork::Private")

CloudNetwork.where(:type => "ManageIQ::Providers::Openstack::CloudManager::CloudNetwork", :external_facing => true).update_all(:type => "ManageIQ::Providers::Openstack::NetworkManager::CloudNetwork::Public")

CloudSubnet.where(:type => "ManageIQ::Providers::Openstack::CloudManager::CloudSubnet").update_all(:type => "ManageIQ::Providers::Openstack::NetworkManager::CloudSubnet")

or delete the orphaned OpenStack networks and subnets:
------------------------------------------------------

CloudNetwork.where(:type => "ManageIQ::Providers::Openstack::CloudManager::CloudNetwork", :ems_id => nil).delete_all

CloudSubnet.where(:type => "ManageIQ::Providers::Openstack::CloudManager::CloudSubnet", :ems_id => nil).delete_all

or delete all orphaned networks and subnets:
--------------------------------------------

CloudNetwork.where(:ems_id => nil).delete_all

CloudSubnet.where(:ems_id => nil).delete_all

Verified in 5.6.1.0 using step 3 in lsmola's comment 'delete all orphaned networks and subnets'

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://rhn.redhat.com/errata/RHSA-2016-1634.html

BZ is closed.