aaa: Change the check of internal command execution

Both canDoAction and executeAction set the isSucceeded flag
to false if they fail.
We should check if AddUser or AddGroup failed not just by their
canDoAction phase, but also by their execution phase.
Hence, using the isSuceeded flag is more appropriate solution.

Bug-Url: https://bugzilla.redhat.com/1121954
Change-Id: Ie7093e3e20387c2cb9d56c818ac4b74d49b07ad1
Topic: AAA
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>