NFS backend for Glance images

Bring support to mount filesystem_store_datadir to a remote NFS share.

Detailed description of this patch:
* add a new backend in cloud::image::api (nfs).
* add a new parameter to configure the NFS device (false by default).
* This parameter is required when using NFS backend, otherwise it will
  fail.
* Mount the NFS share using a new Puppet module by using a hash to
  configure the share.
* Add 4 new Puppet modules for NFS, common, types and rpcbind. The 3
  last ones are dependencies.
  Note: this NFS module has been chosen because it's well tested, quite
  updated and support Hiera data model which will allow us great
  flexibility for complex deployments.
* add unit tests with two new contexts when running NFS backend.