proper location for LogStash::Environment.pattern_path

At some point `LogStash::Environment.pattern_path` was located in the bootstrap code, because of path issues but this creates some problems for code that executes outside the context of logstash, like specs, which requires `LogStash::Environment.pattern_path`.

Some temporary cut&paste fixes have been made to fox this but we need to clean this up. I also think we need to revisit all the code surrounding this `LogStash::Environment.pattern_path` which seems weird at some places.

The problem is detailed in logstash-plugins/logstash-filter-grok#72 and also relates to logstash-plugins/logstash-input-syslog#25 logstash-plugins/logstash-patterns-core#137