revocation_list only call isotime on datetime objects
revocation_list only call isotime on datetime objects

If expires is a str then isinstance six.text_type is false then the following
exception is raised:
AttributeError: 'str' object has no attribute 'strftime'

This change explicitly checks for a datetime object, which is what
timeutils.isotime expects.

Change-Id: I8012f96498fffe5f54e7f1ca08b7beb6fa168f9c
Closes-Bug: #1285871

ajayaa/keystone,mahak/keystone,nuxeh/keystone,MaheshIBM/keystone,jonnary/keystone,jonnary/keystone,dstanek/keystone,vivekdhayaal/keystone,idjaw/keystone,cernops/keystone,roopali8/keystone,jumpstarter-io/keystone,rodrigods/keystone,promptworks/keystone,promptworks/keystone,openstack/keystone,dstanek/keystone,blueboxgroup/keystone,roopali8/keystone,ging/keystone,dims/keystone,UTSA-ICS/keystone-kerberos,rushiagr/keystone,himanshu-setia/keystone,jamielennox/keystone,UTSA-ICS/keystone-kerberos,reeshupatel/demo,JioCloud/keystone,mahak/keystone,JioCloud/keystone,MaheshIBM/keystone,reeshupatel/demo,openstack/keystone,ilay09/keystone,vivekdhayaal/keystone,klmitch/keystone,dims/keystone,takeshineshiro/keystone,rajalokan/keystone,takeshineshiro/keystone,jamielennox/keystone,maestro-hybrid-cloud/keystone,rushiagr/keystone,rajalokan/keystone,dstanek/keystone,himanshu-setia/keystone,cernops/keystone,reeshupatel/demo,nuxeh/keystone,openstack/keystone,nuxeh/keystone,blueboxgroup/keystone,rushiagr/keystone,ging/keystone,rajalokan/keystone,klmitch/keystone,mahak/keystone,ilay09/keystone,idjaw/keystone,jumpstarter-io/keystone,jumpstarter-io/keystone,rodrigods/keystone,vivekdhayaal/keystone,maestro-hybrid-cloud/keystone,ajayaa/keystone,promptworks/keystone,ilay09/keystone