aaa: change code of AddUser and AddGroup

removing the check from canDoAction to check if entity exists
in directory as these checks are done both by rest-api and gui.

Bug-Url: https://bugzilla.redhat.com/1120609
Topic: AAA
Change-Id: If841c8d30d6ab2c5d06b1637cde93cfc65598a96
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>