[TEST] Replaced the "rest-api-spec" submodule with Elasticsearch core repository

Previously, the REST API specification and tests were contained in a separate
repository. This change adds the full Elasticsearch core repository as a
submodule, since it contains the specs and tests since elasticsearch/elasticsearch@63a9ae4e2b17584c98b3a8f4d68d25920899fb4d

Related: elasticsearch/elasticsearch#4540, elasticsearch/elasticsearch#4541
Updated the main Rakefile to reflect adding Elasticsearch core repo as submodule

Renamed and updated the `update` Rake task, added `status` and `changes` tasks.
[API] Updated the path to YAML tests in API integration test runner

Point the YAML runner to the new tests location (main ES repo)
[API] Added `ignore_unavailable`, `allow_no_indices`, `expand_wildcards` parameters to "indices" APIs

Also added missing `master_timeout` parameter to "Optimize" API
[API] Added documentation for the internal `force` and `full` parameters in the "Indices Flush" API
Added the `elasticsearch:build` Rake task

Build Elasticsearch from the master branch:

    rake elasticsearch:build

Build Elasticsearch for a specific branch:

    rake elasticsearch:build[0.90]