ipaclient: Fix issues with FreeIPA 4.6 and prior

check_ldap_conf is only available in FreeIPA 4.7 and later and
tasks.is_nosssd_supported is only available since 4.6.90.pre2.

check_ldap_conf is None (ansible_ipa_client) if it can not be imported.
hasattr has been added to check if is_nosssd_supported is a valid
attribute in tasks.

Fixes: #61 (ipaserver role - Fails on ipaclient install)