Support standard synced folder middleware (fixes various bugs / missing features)

[maxlinc]

I was going to report that vagrant rsync and vagrant rsync-auto commands do not work when using the openstack provider. I'm pretty sure this is one of several bugs / side-effects of what @ggiamarchi wrote in #207:

synced folders in this provider does not use the standard vagrant middleware

I did work to fix this a while back on the Vagrant-Rackspace plugin, along with some other things to make the plugin work better with Vagrant 1.5+. You can see all those changes in mitchellh/vagrant-rackspace#129, but I believe the relevant commit was maxlinc/vagrant-rackspace@15abef8.

[menrcom]

Also, initial sync during vagrant up doesn't work in case if rsync isn't installed in VM image.
Usual behaviour for vagrant is to install rsync if it is missing, as I know. But with openstack provider this doesn't happen.

[ggiamarchi]

Fixed by #295