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.
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