core: Fix DB storing for principal names as provided by Active-Directory

IPA provides principal correctly (format of user@domain).
AD provides only the username.
We would like to present at UI in format of principal name.
Since this issue may occur with other directory services,
I prefered to handle this at the LdapBrokerCommandBase , regardless
of provider type.
The upgrade script is required to handle already existing users.

Bug-Url: https://bugzilla.redhat.com/888736
Change-Id: Ia9f8b60280d76b72a26f0d1feaf2650c4ef1ce3f
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>