Add doc generation tasks to logstash

Fixes #2256
Make the test plugins install throw bundler, so we don't have to install them every time we run the test (it makes the thing slow). Refactor also the bundler install task into a single one, making calls to it when necessary.

Fixes #2237
make sure the bundler task is just used once while bootstraping tests

Fixes #2237
make sure the fail fast test also have the bootstrap for tests done

Fixes #2237
removes the patterns from patterns dir

The core patterns are now provided by the logstash-patterns-core gem.
This empty folder can be used for custom patterns.

Fixes #2228
Refactor doc generation

- Use a generic task for installing all plugins
- Use refactored bundle install task

Fixes #2258
Add comment for locale in date filter.

For non-english environment the date filter don't parse correctly the month name. See : http://stackoverflow.com/questions/26653490/logstash-date-filter-failed-parsing

Fixes #1987
updates docs to fix #2240

Fixes #2243
Small change so apt-transport-https package is not required on Debian. Yum supports https by default.

Fixes #2243