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>