Perf dashboard should be able to list iOS versions as well as OS X versions

Perf dashboard should be able to list iOS versions as well as OS X versions
https://bugs.webkit.org/show_bug.cgi?id=146003

Reviewed by Stephanie Lewis.

Generalized pull-osx.py so that it can run an arbitrary shell command to fetch OS versions based on
information specified in config.json.

* tools/pull-os-versions.py: Renamed from pull-osx.py.
(main): Use available_builds_from_command when 'customCommands' is specified.
(available_builds_from_command): Added. Executes a shell command to fetch a list of available builds.
(fetch_available_builds): Now takes the repository name.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@185578 268f45cc-cd09-0410-ab3c-d52691b4dbfc