pass network configuration through to target for ubuntu and centos
curtin will now query the in-target cloud-init for the NETWORK_CONFIG_V2 feature flag. If present, curtin will copy in any custom networking config an defer rendering to cloud-init upon first boot. This function is also implemented for Centos/Enterprise Linux distros.
Centos network curthooks has the following capabilities - Only runs when 'centos_network_curthooks: true' is passed in via curtin config. MAAS will modify centos built-in curthooks to use the centos_network_curthooks as needed. - If the target centos image does not contain a new enough cloud-init then the cloud-init-el-stable repo is configured and curtin installs epel-release, cloud-init and bridge-utils if needed
Additional VMtests were added for Centos66 and Centos70 covering all of the network tests. Note that network_mtu is disabled as mixed v4 and ipv6 mtu settings are not supported in sysconfig.
Other useful features in this branch: - tools/build-deb now lets users set the RELEASE value like: RELEASE=xenial ./tools/build-deb and the RELEASE value will be set in the changelog for the deb