Make RGW configurable in Vagrant

Depending on what we want to test, it might be handy to simply disable
the creation of a specific virtual machine. Now if NRGWS is set to 0
this will disable it.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #132 from leseb/rgw-vagrant

Make RGW configurable in Vagrant