Add new lock level for node allocations

The new lock is similar to the BGL in the sense that it has its own
level and there is only one. It is called “node allocation lock”.
Logical units will use it to synchronize with instance creations, which
in turn will start using opportunistic locks on nodes.

Additionally, an assertion in GanetiLockManager gained a message.

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