Add list of domains associated to our realm to cn=etc

Add new LDAP container to store the list of domains associated with IPA realm.
Add two new ipa commands (ipa realmdomains-show and ipa realmdomains-mod) to allow
manipulation of the list of realm domains.
Unit test file covering these new commands was added.

https://fedorahosted.org/freeipa/ticket/2945
ipasam: use base scope when fetching domain information about own domain

Since we use associatedDomain attribute to store information about UPN suffixes
and our own domain, searching subtree is going to return more than one entry.
Limit search for own domain by base scope as we only need to fetch our own
domain information here, not UPN suffixes.

Required for https://fedorahosted.org/freeipa/ticket/2945