Allow overriding of minion ip in local-up-cluster.sh

Allow specifying $MINION_IP as an environment variable for use by
local-up-cluster.sh so you can test Kubernetes services when running a
simple local cluster. The default value of 127.0.0.1 is routable by
containers (at least, that doesn't route to the host, which is what's
needed), so this allows the user to specify an alternative (e.g. the IP
address of the local NIC).
Add registry support for builds

Add docker_builder_image option to build-controller to allow you to
switch the image used for docker builds.

Add docker_registry option to build-controller to allow you to specify
the docker registry to which builds should push their images.

Add ImageTag field to BuildConfig.
Merge pull request #13 from ncdc/build-push-to-registry

Build push to registry
Introduce pluggable strategies for build types