Remove vestiges of Assignments from LDAP Identity Backend

Bug 1204995

Change-Id: Ife92041ffc386b8f9629096ebd65020f3cc7ae26
Scipped tests don't render as ERROR's

Replaced `raise nose.exc.SkipTest()` statement to self.skipTest().
Removed unused nose.exc imports.

Fixes bug 1172794

Change-Id: Ieb353864acadef43508d185156c7fa1667baa845
Merge "Scipped tests don't render as ERROR's"