OpenStackCloud.create_server fails if seucurity_groups=None

[Alvaro Aleman]

Currently, OpenStackCloud.create_server() fails if the 'security_groups' kwarg is None, because all input to to 'security_groups' that is not a list itself is put into a list: https://github.com/openstack-infra/shade/blob/9b22e1cea6b0a802a151a5a75501660ade71ab6a/shade/openstackcloud.py#L4995

This is a compatibility breach to the shade version that is on pypy (1.13.2) and breaks some code that depends on shade, e.G. here: https://github.com/metacloud/molecule/blob/c745c35a0198f774a0e9006954cc3b5707232314/molecule/driver/openstackdriver.py#L124

Shade should only put the input to security_groups in a list if it is not none or a list.

[Alvaro Aleman (alvaroaleman)]

Reported on storyboard, as Shade doesn't seem to do any bugtracking here: https://storyboard.openstack.org/#!/story/2000828