RemoteFS: Use nas_ip and nas_share_path options

RemoteFS: Use nas_ip and nas_share_path options

This replaces the <x>fs_shares_config file configuration
options with the existing nas_ip option and new
nas_share_path and nas_mount_options configuration
options.

This means that RemoteFS drivers will manage a single
export rather than a handful of unrelated exports.

If the nas_ip and nas_share_path options are set, they
are used.  If not, the previous configuration mechanism,
based on loading a set of shares from a file configured
by <x>fs_shares_config will be used.

Also use nas_mount_options to replace
nfs_mount_options for consistency.  If nas_mount_options
is not set, nfs_mount_options will be used for
compatibility.

Implements blueprint: remotefs-share-cfg-improvements
DocImpact: new configuration options

Change-Id: Ic72ae6c7fdd2c6a7fea27152f27c6521a561977b