Add [assignment].driver to sample config
Add [assignment].driver to sample config

There's a new config option [assignment].driver that wasn't included
in the sample config file. This makes it more difficult than necessary
for deployers to configure.

Fixes bug 1202778

Change-Id: I04b09c214b9ea997d8f540a72978ce9b19b4138d

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