Set the default metavg to be equal to the cluster name

The default metavg was always set to be the constant value "xenvg".
This is OK for most cases, but if the cluster is initialized with
a default name through the --vg-name option, the metavg should change
as well.

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