core: Added validation on managed custom mount option

When adding a new NFS/Posix storage server connection, if the custom mount
options contains a managed option (i.e. property that the engine is managing
directly), the command will fail with a CDA

Change-Id: Icc5a74379de55aa81194298be634f689efd331d5
Bug-Url: https://bugzilla.redhat.com/1129597
Signed-off-by: Tal Nisan <tnisan@redhat.com>