daemon-util: Add function to start and stop all daemons

This is, to some degree, duplicated code from the init script. However,
the init script has to conform to standards of the underlying Linux
distributions, while these functions will be called by Ganeti itself. By
moving more code into daemon-util, the amount of duplication has been
reduced.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>