Disable agent bond by default, remove AB_OFF

[s-spindler]

Thanks a lot for this great project!

We would like to create a base image built on top of the fabric8 image for several of our services that has the agent disabled by default.
The problem: if we set AB_OFF in our base image there is no way to enable it again in one of the extending images.

What is worse we also cannot unset AB_OFF at runtime for when we want to start a container with the agent enabled.

Why not simply disable the agent by default and only enable it if AB_ENABLED is set to some value?