core: Check validity of new storage connection mount options ignoring case

When adding a new NFS/Posix storage server connection, the custom mount
options should be validated case insensitively

Change-Id: Ie5334bdef5955a9b48e266d8dae7ecb339ccc376
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1141500