ipareplica: Use ipaserver_realm as a fallback for realm

Use ipaserver_realm as a fallback if ipareplica_realm is not defined. This
had been done for ipareplica_domain and ipaserver_domain, but was missing
for ipareplica_realm and ipaserver_realm.

Related: #114 (ipareplica 'Env' object has no attribute 'realm')