Add error code for temporary lack of resources

When an instance creation uses opportunistic locks, the iallocator might
not be able to find an allocation solution if not enough node locks (or
a suboptimal subset thereof) were acquired. As per the design document
<doc/design-opportunistic-locking.rst> a new error code denoting
temporary failures is added.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>