sss_cache: support for subdomains

https://fedorahosted.org/sssd/ticket/1741
sss_cache: Remove annoying messages

When requested entry was not found in one domain, an ERROR message
was written to the user even if the entry was found in
the next domain and deleted properly.
Fix simple access group control in case-insensitive domains

https://fedorahosted.org/sssd/ticket/1713

In the simple access provider, we need to only canonicalize user names when
comparing with values in the ACL, not when searching the cache. The sysdb
searches might do a base search with a DN constructed with the username
which fails if the username is lower case.