Allow specifying vos create/addsite volume IDs

Allow specifying vos create/addsite volume IDs

This adds the -id option to 'vos create', and the -roid option to 'vos
create' and 'vos addsite'. This allows the user to manually specify the
volume IDs that a new RW or RO volume will get (or explicitly specify
that an RO volume ID should be unset), instead of always relying on the
volume IDs retrieved from the vlserver.

Reviewed-on: http://gerrit.openafs.org/157
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>