Configurable assignment driver

Allow the [assignment] backend to be switched (eg; to LDAP) for
http://docs.openstack.org/developer/keystone/configuration.html#read-only-ldap

Change-Id: I9e4d5a4f4538025c55ebd7db39a78c5135af48e1
Closes-Bug: #1318992
Add TLS options for [ldap] to keystone.conf

Change-Id: I27d27042ca96860e8f04c0f51639aa4e435007b3
Closes-Bug: 1322146