Fix a number of KVP parameter parsing issues:

1) we should not be separating the prefix and instance numbers with an '_',
2) IPADDR/NETMASK instance 0 does have a suffix which we do not provide,
3) GATEWAY instance 0 is inconsistant,
4) IPv6 should be configured whether IPv4 is DHCP or not, and
5) DHCP mode is selected via BOOTPROTO=dhcp not DHCP=yes.

BugLink: http://bugs.launchpad.net/bugs/1540586Signed-off-by: Andy Whitcroft <apw@canonical.com>