Allow programs to be part of the Ganeti library

Eventually this will help ensuring that clients and servers are of the
same version, as long as they're imported from the same path. Currently
it's relatively easy for gnt-* and ganeti-* to be from a different
version.

Scripts will be at ganeti.client.gnt_* and a small bootstrap script
calls a “Main” function from the module.

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