Add user authentication by userid for Ldap user provisioning.

Since at that moment of provisioning the user has already logged in and is defined in the VMDB.

https://bugzilla.redhat.com/show_bug.cgi?id=1139763
Add test cases for User.find_or_create_by_ldap_attr.

https://bugzilla.redhat.com/show_bug.cgi?id=1139763
Merge pull request #713 from lfu/ldap_dn_provision_after_logon_1139763

Add user authentication by userid for Ldap user provisioning.