IPA: Add ipa_parse_search_base()
    
    Previously, we were using sdap_parse_search_base() for setting up
    the search_base objects for use in IPA. However, this was
    generating unfriendly log messages about unknown search base
    types. This patch creates a new common_parse_search_base() routine
    that can be used with either LDAP or IPA providers.
    
    https://fedorahosted.org/sssd/ticket/1151