Allow the docker image tags to be overridden in TravisCI

[jszwedko]

Will allow forks to more easily publish images.

To ease development of the separate Citygram front-end mentioned in https://github.com/codeforamerica/citygram/pull/273#issuecomment-403101644, it'd be useful to push separate images while our changes are integrated upstream.

[bigfleet]

@jszwedko I think in the cases where these aren't set, it'll try to push to -citygram --> probably worth our (your, if I might be so bold!) time to write an actual script that determines and runs the correct command, with the above lines serving as the basis for the script in question. (I think we can already assume that Ruby is available, so as long as you stick to stdlib, you/we could write it in ruby if that helped.)

[jszwedko]

@bigfleet thanks for taking look!

I'm not sure I follow when the case would occur where you'd end up with -citygram. What I have here is relying on bash variable expansion to default the value if the variable isn't set.

For example:

$ echo $DOCKER_IMAGE_DATA

$ echo ${DOCKER_IMAGE_DATA-citygram/data}
citygram/data
$ export DOCKER_IMAGE_DATA=foo
$ echo ${DOCKER_IMAGE_DATA-citygram/data}
foo

It's certainly possible that I'm missing something though 😄 More than happy to update this.