vagrant: add support for Linode VMs

vagrant: add support for Linode VMs

Other things of note:

o You can now set the ceph branch to test against in
  vagrant_variables.yml.

o You can now set the ceph_conf_overrides in vagrant_variables.yml.

This commit depends on an open PR:

https://github.com/displague/vagrant-linode/pull/66

Until that is merged, you must copy the changed file to your copy
of the vagrant-linode plugin, e.g.:

    cp lib/vagrant-linode/actions/create.rb ~/.vagrant.d/gems/gems/vagrant-linode-0.2.7/lib/vagrant-linode/actions/create.rb

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>