ipapwpolicy: Use global_policy if name is not set

If the name is not set, the policy global_policy is now used. It was needed
before to explicitly name the global_policy. Also a check has been added
to fail early if global_policy is used with state absent.

The README for pwpolicy has been extended with an example for global_policy
and also the description of the name variable.

The test has also been extended to check a change of maxlife for
global_policy and that global_policy can not be used with state: absent

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1797532