feat(compose): Add ability to use custom images for any Sentry service (#602)

This change allows one to override any Sentry service image, mostly for testing purposes. It also removes the SENTRY_VERSION variable as docker-compose makes it very hard to cascade default values for these. Next step is to have integration tests in getsentry/snuba and getsentry/relay (and possibly for getsentry/symbolicator) for getsentry/onpremise using this PR.

Also related: #596.