Allow to use non-Str attributes as keys for members
Locations use DNSNameParam as pkey_value, but implementation of searches
for members was able to use only Str param. This commit allows to use
other param classes for search.
Required for: https://fedorahosted.org/freeipa/ticket/2008
Reviewed-By: Petr Spacek <email@example.com>
Reviewed-By: Jan Cholasta <firstname.lastname@example.org>