Fix the docstring of _default_security_group
Fix the docstring of _default_security_group

Since I3879ae24e47590d12fb09a4bb4697f139f57caf7,
DeletableSecurityGroup has been removed and the return value of
_default_security_group() has been changed.
So this patch fixes the corresponding docstring.

Change-Id: I45ec199eb80b5687d38d1a1b0f561277f8e3b37c

masayukig/tempest,vedujoshi/tempest,sebrandon1/tempest,Juniper/tempest,Tesora/tesora-tempest,openstack/tempest,vedujoshi/tempest,sebrandon1/tempest,cisco-openstack/tempest,Tesora/tesora-tempest,Juniper/tempest,openstack/tempest,masayukig/tempest,cisco-openstack/tempest