Decouple build and template

[jorgemoralespou]

Split the nexus3.json file into 2. One that will build the image locally if required.
Another that will get the built image from Dockerhub, as this can be easier.

Set up automated builds so that if you solve #1 a user can pull directly a Red Hat compliant nexus from dockerhub and not have to build their own. Only in case where they need to base it in RHEL there should be this requirement.

You can do also:

• Provide just a template that will pull down the centos based nexus from dockerhub
• Provide a template + build config for building the RHEL nexus locally