If an ident member of an IdentKeyVal relationship starts with no_ or -,
handle it the same way we do for a key. Some unittests are added to
check that check_ident_key_val behaves as expected.

This patch also changes ForceDictType to, for now, fail on such an
entry, and the same to happen when creating an instance or modifying its
nics or disks.

This behavior will be used later on to allow deletion of os entries in
os parameters.

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