Add a bind_host parameter to support Openshift integration (localhost
can't be used)
Add IS_OPENSHIFT variable so that virtualenv installation can take
--system-site-packages param
Better handling of external clusters in Elasticsearch check.

- Add `is_external` option to config which will skip the node hostname
  and local hostname check.
- Refactor hostname check into separate function for clarity.
- Remove unecessary closures for clarity.
compress dogstatsd payloads
only compress dogstatsd payloads that are larger than 1k
correctly initialize self.current_mbc
rename function so nosetests does not run it
Add the possibility to select the http client to use in the forwarder

Default to the simple http client as there is a bug when using the curl
one with ELBs.
Merge pull request #883 from DataDog/es-external-node

Better handling of external clusters in Elasticsearch check.
Merge branch 'issue894'
Merge pull request #893 from DataDog/dogstatsd-compression

Dogstatsd compression
Prepare changelog for 4.2.1
Comment the bind_host line to reflect that this is optional
Merge branch 'master' of https://github.com/DataDog/dd-agent
Fix release date for 4.2.1
Merge openshift PR
Use a copy of the tags to avoid adding tags to the same list at each iteration of the check
Update CHANGELOG.md
Pimp the changelog
Try to start the agent using supervisor if supervisor is already running