[fuel] Invalid handling of node without disks

1. Install Fuel master
2. Create a slave node without disks
3. Add this node to created environment
4. "?GB" displays instead "0GB" on node (see first screen, node (8a:a8))
5. Also I think that if user wants to configure disks for this node, we need to display something like "No disks to configure" (see second screen)

"build_id": "288", "build_number": "288", "release_versions": {"2015.1.0-7.0": {"VERSION": {"build_id": "288", "build_number": "288", "api": "1.0", "fuel-library_sha": "121016a09b0e889994118aa3ea42fa67eabb8f25", "nailgun_sha": "93477f9b42c5a5e0506248659f40bebc9ac23943", "feature_groups": ["mirantis"], "fuel-nailgun-agent_sha": "d7027952870a35db8dc52f185bb1158cdd3d1ebd", "openstack_version": "2015.1.0-7.0", "fuel-agent_sha": "082a47bf014002e515001be05f99040437281a2d", "production": "docker", "python-fuelclient_sha": "1ce8ecd8beb640f2f62f73435f4e18d1469979ac", "astute_sha": "a717657232721a7fafc67ff5e1c696c9dbeb0b95", "fuel-ostf_sha": "1f08e6e71021179b9881a824d9c999957fcc7045", "release": "7.0", "fuelmain_sha": "6b83d6a6a75bf7bca3177fcf63b2eebbf1ad0a85"}}}, "auth_required": true, "api": "1.0", "fuel-library_sha": "121016a09b0e889994118aa3ea42fa67eabb8f25", "nailgun_sha": "93477f9b42c5a5e0506248659f40bebc9ac23943", "feature_groups": ["mirantis"], "fuel-nailgun-agent_sha": "d7027952870a35db8dc52f185bb1158cdd3d1ebd", "openstack_version": "2015.1.0-7.0", "fuel-agent_sha": "082a47bf014002e515001be05f99040437281a2d", "production": "docker", "python-fuelclient_sha": "1ce8ecd8beb640f2f62f73435f4e18d1469979ac", "astute_sha": "a717657232721a7fafc67ff5e1c696c9dbeb0b95", "fuel-ostf_sha": "1f08e6e71021179b9881a824d9c999957fcc7045", "release": "7.0", "fuelmain_sha": "6b83d6a6a75bf7bca3177fcf63b2eebbf1ad0a85"