Add Docker build argument to specify pip version and dh-virtualenv branch

In st2/Makefile we do a neat trick to grab the version of virtualenv to install from the requirements.txt file instead of specifying the version in multiple places.

We could do a similar thing in our Dockerfiles to specify the version of pip, pulling it from st2/Makefile via a well crafted curl/grep/sed command. In such a case, when it comes time to update the version of pip, we could add a build argument to Dockerfiles that specifies the branch of ST2 to pull from.

Alternatively, we could just add a build argument that directly specifies the version of pip to pull, but that still requires us to update the default version every time we rev pip.

We could also add build arguments to specify the branch of dh-virtualenv, just in case we need to make any more changes to our fork.

Opening this issue to start the discussion, and keep track of progress.

Start with st2/Makefile around the existing virtualenv version lookup, then inspect the repository's Dockerfiles and their pip and dh-virtualenv setup. The work is complete when the chosen build-argument approach is agreed and the Dockerfiles can use it to select the pip version and dh-virtualenv branch without duplicating version information.