engine: Can't add user without system level admin permission

This patch fixes a regression introduced at Ib62e1c051bc78b8a9ec0f32e6ba4eb9484242591

1. Introducing a new permission of adding non existing users - this is required, as the other options are:
a. Using the existing "manipulate users" permission - this will cause a regression, allowing roles to add users directly without the need to use the add permissions dialogs.
b. Removing the permission check for non existing users/groups - this will break https://bugzilla.redhat.com/923100

2. The new permission is added to all the roles that have manipulate permissions manipulation -
this is done in order to fix the above regression.

Change-Id: I308f9cc5edb53b9633d768fd3d382dc9cf62031c
Bug-Url: https://bugzilla.redhat.com/1070651
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>
Signed-off-by: Ravi Nori <rnori@redhat.com>