Fix migration plugin compat check
    
    Ticket #2274 implements a check for compat plugin and warns user if
    it is enabled. However, there are 2 issues connected with the plugin:
    1) The check is performed against the remote (migrated) LDAP server
       and not the local LDAP server, which does not make much sense
    2) When the compat plugin is missing in cn=plugins,cn=config, it
       raises an error and thus breaks the migration
    This patch fixes both issues.
    
    https://fedorahosted.org/freeipa/ticket/2508