Adds SSH communication to HNAS drivers

Adds SSH communication to HNAS drivers

This commit adds SSH communication to HNAS iSCSI and NFS drivers. The user now
has the option to export the server public keys to HNAS and configure the
authentication via public keys. This avoid exposing HNAS passwords in the plain text
config file and exposing the private HNAS internal network.

Implements: blueprint hds-hnas-ssh-backend
Change-Id: Iafb5d7a4b8607128437b4e606e9ed478c24a43a0