modules.opnfv: fuel adapter: Switch to MCP

modules.opnfv: fuel adapter: Switch to MCP

The current codebase was intended for Fuel@OPNFV up to and including
the Danube release (based on Fuel@Openstack).
Starting with the Euphrates release, Fuel@OPNFV is based on Mirantis
Cloud Platform (MCP), so the Fuel adapter in modules.opnfv needs a
rework to accomodate the new platform:
- remove unused obsolete code in opnfv.utils;
- switch to key-based SSH auth instead of password-based;
- drop proxy-ing SSH to cluster nodes via installer node, instead
  SSH directly via PXE/admin network;

Notes:
- MaaS node (when present) is marked with 'installer' role to prevent
  ovs-vsctl queries on it;
- gtw01 node (when present) is not marked with any role yet;

JIRA: FUEL-394

Change-Id: If2b7744b950cd502df8eaca0137c9f845a9d077d
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>