Separate Dockerfiles from main codebase

For both access and release process reasons, having the Dockerfiles for both Couchbase Operator and Couchbase Admission Controller is a highly desired need. There's basically two parts:

1. Move Dockerfiles to new locations: couchbase/docker repo for the Dockerhub ones, and couchbase-partners/redhat-openshift for the OpenShift ones
2. Modify the Dockerfiles and the processes used in the various build jobs to utilize the new locations:
   • The Dockerfiles should contain args that can be set via the command line to override the defaults, depending on the build (e.g. able to pull from latestbuilds instead of S3 for the source/artifacts)
   • The files should probably also be auto-generated as with the current Server and Sync Gateway process

The 1.2 release will need to be done manually for now, but for the following release we should have this new setup and process in place.