Make NFS exports in generic driver permanent

Generic driver uses 'exportfs' utility for exports handling, but it stores
exports in temporary file /var/lib/nfs/etab and server loses all exports after
reboot. This change copies temp exports file to permanent exports file located
in /etc/exports after each action with exports and makes reexport of all
shares. After restart of server all exports will be restored from permanent
file.

Change-Id: I6a1e7074f18c70df5785d236c6f70c40731cda77
Closes-Bug: #1358688
Merge "Make NFS exports in generic driver permanent"