New scan plugin for Proxmox 2 and 3.

Apart from said plugin (and changes required by it), this pull request contains also some additional changes, i.e.:
- 'force autoscan' is now triggered only when SNMP name is missing (see: `src/ralph/scan/manual.py`);
- improved disk shares' matching with their pvs (see: `src/ralph/discovery/hardware.py`).