Prevent ssconf values from having non-string values

For whatever reason, my test cluster managed to acquire
shared_file_storage_dir with a None value, instead of empty
string. This is not flagged in masterd itself, but the node daemon
will fail in writing the value to disk, as it calls len() on the
received value.

Since this is a bad case, we should detect it as soon as possible (we
basically shouldn't be able to set it), but in the meantime we at
least prevent ssconf writes with such values.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>