Use omnibus bundling

This PR is a first step towards using Chef Omnibus to build the agent.
It will allow us to bundle all the required dependencies (including python) in the agent package which will solve dependencies issues that have been encountered in the past as the agent will work in its own sandbox in /opt/datadog-agent

What does this PR do:
- Remove dependencies that were bundled with the agent code (ntplib, yaml, boto...)
- Migrate postgresql check and mysql check to use pure python dependencies
- Remove (some) compatibility code
- First pass to a system checks that use psutil

This PR is not ready for merging yet and still needs more work but the PR can be useful for discussion.


Worth standardizing on using requests in a later release