ipa[client,server]: Inspect validate_domain_name for 4.6 and prior

The entity argument for validate_domain_name is only available in
FreeIPA 4.7 and later. This has been fixed using inspect to be able to
detect if entity is a valid argument. If not the whole realm name check
is skipped.

Related: #61 (ipaserver role - Fails on ipaclient install)
Fixes: #66 (Python 2 error with validate_domain)