ipa[server,replica]: New variables to set firewalld zone

The new variables ipa[server,replica]_firewalld_zone have been added to
be able to set the zone in which the needed services for IPA are enabled.

New tasks have been added to check if the zone is available in the runtime
and also permamanet environment.

The code to enable firewalld has been moved out of thee
ipa[server,replica]_install_packages blocks to make sure that the firewalld
service is also enabled if the package is already installed.

Fixes: issue #177 (How to set up firewalld zones?)