Fixed handling of the display-manager service when changing the remote administration support

- bnc#893606
Added test case
Version and changes
Using function IsEnabled() instead of a variable
Extended test case
Implemented Ancor's recommendations from review

- Using let/allow instead of expect
2 vs 1 -> no Expect in Before
Comments from review
Merge pull request #250 from kobliha/vnc

Configuration of remote administration has been fixed to reload/restart services and/or warn user if necessary