ES common index

This will allow as to use parent/child relationship in ES. Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/375

• Fix settings overlapping
• Specs
• Test everything
• Change rake tasks
• Change index settings for all models
• Update the install doc
• Update the update doc (I created https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5545)
• Prepare text for release blog post

Proposed addition to releasepost:

We totally changed the structure of Elasticsearch index this is why we need to remove whole index and build new one. To be able to use parent/child relationship in ES we decided to move everything in a single index. This solution has some drawback - if we decide to change the type of some existing field in ES we will need to rebuild whole index for every entity. So it makes it harder to support but we expect to have more performance advantages with parent/child relationships.

Migration

Remove old indexes

curl -XDELETE 'http://localhost:9200/_all/'

Build new indexes as described in Elasticsearch integration