Fix config.template.py

The config.template.py file contains a line: HOST_USER = 'tango' with a comment suggesting that this variable is used for the distDocker setup. Searching the repo, however, suggests that this is a typo. The variable actually used by vmms/distDocker.py is not HOST_USER, but rather DOCKER_HOST_USER.

Thus, I suggest removing the HOST_USER line from config.template.py and updating the comment to indicate that for distDocker, both HOST_ALIAS and DOCKER_HOST_USER should be set.

It seems like this change should also be suggested with the upstream cmu repo.

Related: deployment#2