Skip vagrant disksize option if no plugin.
Skip vagrant disksize option if no plugin.

Previously, `vagrant up` would bail if no `vagrant-disksize` plugin was
installed. This option is just a nice-to-have, so skip it rather than
bailing.

pgbackrest/pgbackrest,pgmasters/backrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest