Add uuid on node/instance add and cluster init

This patch does a little bit of cleanup first, since we want to call
GenerateUniqueID without reacquiring the lock.

Note that we don't necessarily need to do this for the cluster, since at
first startup ConfigWriter will do it anyway. But it's better to
explicitely do this instead of relying on the automated upgrade.

Additionally this patch adds ctime/mtime population at cluster init
time. mtime is not necessarily needed (master will update it
automatically, but we're doing it anyway for consistency).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>