refactoring add/remove account and user tests

These tests cover account registration and de-registration
user creation/delete, and listing of both account and user.

Some functions were renamed to better represent their purpose
and make them generic enough to be used by other tests

Change-Id: Ie622daccfc8e2d2fb45565952a99d7d832ce1189
Signed-off-by: Thiago da Silva <thiago@redhat.com>
Reviewed-on: http://review.gluster.org/6212
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/6257