ipareplica: Support inventory groups.ipaserver

Altough most of ansible-freeipa documentation and playbooks use
'ipaserver' as the group for the first server deployed for a realm, the
ipareplica role only supported the use of groups["ipaservers"] as an
alternative to set ipareplica_servers.

Also supporting groups.ipaserver, as already supported by the ipaclient
role, make ansible-freeipa playbooks more consistent and current
documentation and examples easier to follow when deploying a cluster
with a server and a replica.