Accept other MAC address formats.

This will accept formats like 00006C2830d5 which other backends accept, as well as any other variant that  contains hex digits, such as Cisco's 0000.6C28.30d5.

Bug: 208
Merge pull request #209 from joejulian/accept_other_mac_formats

Accept other MAC address formats.
Honor the auto_config configuration option to allow vagrant to skip configuring network interfaces.
Merge pull request #210 from joejulian/honor_auto_config_network_option

Honor the auto_config configuration option