Implements an NFS backup service in accord with the NFS backup driver
blueprint spec (https://review.openstack.org/#/c/130858).
Currently the NFS backup driver inherits directly from the abstract
chunked backup driver. It should be straightforward to abstract out
of this code a posix filesystem backup driver, without dynamic mount
capabilities, which itself inherits from the chunked backup driver.
When that is done, most of the code in this implementation can be
removed and it can be modified to inherit from the posix filesystem
backup driver itself.