Nail prettytable test dependency at 0.5.0.
Nail prettytable test dependency at 0.5.0.

Prettytable 0.6.0 has a bad md5sum in PyPi and as such causes tests
to fail when creating a new venv. Nail the version to 0.5.0.

Fixes LP Bug #1000757.

Change-Id: I121ba8d411e47c2fe154370d9c3c941e25331ce6

vivekdhayaal/keystone,dstanek/keystone,ajayaa/keystone,vivekdhayaal/keystone,rushiagr/keystone,promptworks/keystone,openstack/keystone,cernops/keystone,nuxeh/keystone,ntt-sic/keystone,cloudbau/keystone,jamielennox/keystone,rajalokan/keystone,ging/keystone,rickerc/keystone_audit,dsiddharth/access-keys,kwss/keystone,townbull/keystone-dtrust,cbrucks/Federated_Keystone,blueboxgroup/keystone,rushiagr/keystone,roopali8/keystone,jonnary/keystone,cernops/keystone,vivekdhayaal/keystone,townbull/keystone-dtrust,ilay09/keystone,rushiagr/keystone,jamielennox/keystone,derekchiang/keystone,klmitch/keystone,citrix-openstack-build/keystone,nuxeh/keystone,rodrigods/keystone,townbull/keystone-dtrust,reeshupatel/demo,cloudbau/keystone,dims/keystone,derekchiang/keystone,himanshu-setia/keystone,ntt-sic/keystone,cbrucks/Federated_Keystone,rodrigods/keystone,klmitch/keystone,UTSA-ICS/keystone-kerberos,roopali8/keystone,jonnary/keystone,JioCloud/keystone,ilay09/keystone,ilay09/keystone,maestro-hybrid-cloud/keystone,UTSA-ICS/keystone-kerberos,savi-dev/keystone,cloudbau/keystone,ajayaa/keystone,savi-dev/keystone,MaheshIBM/keystone,rajalokan/keystone,reeshupatel/demo,maestro-hybrid-cloud/keystone,mahak/keystone,openstack/keystone,rajalokan/keystone,dsiddharth/access-keys,citrix-openstack-build/keystone,savi-dev/keystone,rickerc/keystone_audit,takeshineshiro/keystone,nuxeh/keystone,reeshupatel/demo,promptworks/keystone,cbrucks/Federated_Keystone,promptworks/keystone,kwss/keystone,ging/keystone,kwss/keystone,dsiddharth/access-keys,jumpstarter-io/keystone,mahak/keystone,blueboxgroup/keystone,mahak/keystone,rickerc/keystone_audit,derekchiang/keystone,idjaw/keystone,jumpstarter-io/keystone,ntt-sic/keystone,JioCloud/keystone,idjaw/keystone,openstack/keystone,dstanek/keystone,jumpstarter-io/keystone,takeshineshiro/keystone,citrix-openstack-build/keystone,dstanek/keystone,MaheshIBM/keystone,himanshu-setia/keystone,dims/keystone