Default the VAGRANT_VERSION to ${REVISION}-${BRANCH} if
not set, which expands to '11.0-CURRENT', for example.

If the branch is -CURRENT, -STABLE, or -PRERELEASE, suffix
the VAGRANT_VERSION with the snapshot date.

MFC after: 3 days
X-MFC-Needs: r284893, r284895, r284896, r284897, r284942
Tested with: head@r284961 (patched)
Sponsored by: The FreeBSD Foundation