Do not add user in bugzilla group if the user cannot log in
Accounts that are terminated on bugzilla.redhat.com are configured so
that they cannot be used to log in into bugzilla.
So we should not try to add to bugzilla groups, account that are not
allowed to log in.
python-bugzilla has a handy ``.can_login`` attribute that we can rely on
for this.
This commit thus prevents any account that are terminated from being
added to groups on bugzilla.
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>