Allow start/stop of multiple instances at once

This is an initial version of the multi-instance start/stop, which
allows the gnt-instance startup and shutdown subcommands to work on
multiple instances at once.

The available selections are:
  - "gnt-instance startup --instance name..." which is the default, and
    if passed only one argument it behaves like the original command)
  - "gnt-instance startup --node name..." will work on all instances
    having either primary or secondary one of the passed names
  - "gnt-instance startup --primary name..." will work on all instances
    having primary node one of the passed names
  - "gnt-instance startup --secondary name..." will work on all
    instances having as secondary node one of the passed names
  - "gnt-instance startup --all" will work on all instances

This allows a simple cluster-wide shutdown or node shutdown (or
startup).

Note that this version stops at the first error - it will not try to
start the 2nd instance selected if there is a critical error with the
1st one.

Also note that this version is not faster than simply running manually
"gnt-instance shutdown NAME", because it doesn't parallelize the
shutdown/startup actions.

Reviewed-by: imsnah,roman.marxer