Refactored 'interfaces' role

- from static set of interface configurations, 'interfaces' role has
  been modified to support different sets of default configurations, or
  configuration written by the admin and stored in inventory variables;

- example configurations are provided in the vars/ subdirectory;

- by default 'interfaces' role supports 1 or 2 Ethernet interfaces with
  bridges configured via DHCP (easy integration with virtual machines or
  containers);