Add alpine variants

[tianon]

Closes #41 (changes carried here -- see first commit)

This includes updates to generate-stackbrew-library.sh, and improves consistency both between Debian and Alpine, and between these Alpine variants and other docker-library Alpine variants. 👍

[tianon]

Argh, assumptions in the test we wrote breaking Travis here -- I'll take a closer look at the tests tomorrow to see if there's a simple fix. 👍

[tianon]

docker-library/official-images@1c9ec7d

https://travis-ci.org/docker-library/logstash/builds/181799642 👍

[mi-hol]

@tianon I'm a bit lost because #2429 is flagged as merged but
https://github.com/docker-library/repo-info/blob/master/repos/logstash/tag-details.md#logstash502-alpine (need to copy URL string manually due to git bug!)
contains 'does not exist (yet?)'

1. Any hint how/where I can get the latest alpine based logstash version?
2. the used tag 'alpine' seems to mean 'latest-alpine', to clarify this purpose I'd recommend to rename it

[tianon]

@mi-hol they should be up now -- they do take a little time to build/push after we merge

The alpine tag is the Alpine variant which corresponds to the latest tag. The reason we don't call it latest-alpine is that latest in Docker is really a misnomer to begin with. Something like default would've been more accurate, since it's simply the tag that is implied if no tag is specified. That being said, we don't recommend that users use :latest (and instead recommend that they depend on a specific version or release track, such as logstash:5.0 or logstash:5), and only provide :latest for the case of not specifying a tag at all, ie, unadorned logstash. Thus, the Alpine-based variant of the logstash image is logstash:alpine, similar to how logstash:5 becomes logstash:5-alpine.