Packaging clean-up + SSL warning in http_check

Fixes #768, fixes #769 and fixes #770.

1. `dd-agent` user does not need a shell anymore.
1. `dd-agent` user is deleted when the package is uninstalled.
1. SSL certificate validation for `http_check` is left to `False` (in which case it issues a warning). It can be set to `True` in `http_check; it acts as a control. The default is to keep certificate validation disabled to remain compatible with previous versions.
1. Creates a Vagrant testbed (debian + redhat) that I used for validation.

Tested on Debian 7.1 and CentOS 6.5.