Move HooksMaster out of the mcpu module

We need to do this, so that backend.py doesn't need to import mcpu, and
thus indirectly cmdlib. This reduces the size of the node daemon by
about half, which is very important as it is pinned in memory.

This solves Issue 419.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>