Delete unused _admin_client variable
Delete unused _admin_client variable

_admin_client variable in common/keystone.py isn't used anywhere.
So let's remove it!

Change-Id: I4cea8bb5654f74c6f1091bfd5e9e114701a23ccb

openstack/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum