Change "Next" to "Continue" on storage_vol_page

For consistency, the button to the next page should have the same
label on all pages.

Change-Id: I0c07642ee14d3698852ad6cf2f1c0ed63e73e34e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1053418
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>