Title: citrix_sessions: Handle not set thresholds on single values correctly
Level: 1
Component: checks
Class: fix
Compatible: compat
State: unknown
Version: 1.2.7i1
Date: 1427355497

When a user sets citrix_sessions_default_levels to a value which misses one of
the needed thresholds, the check results in an exception. This is now more
robust.

However, the check should better use the factory defaults mechanism.
