Merge pull request #128 from vkhatri/master

reverting PR #127 and apt preference fix
Use opscenter package instead of opscenter-free.

These packages were merged together after OpsCenter 4.0 was released:
http://www.datastax.com/documentation/opscenter/5.0/opsc/release_notes/opscReleaseNotes40_r.html
Merge pull request #129 from sethrosenblum/opscenter_package_update

Use opscenter package instead of opscenter-free.
Move agent config to /var/lib/datastax-agent/conf/address.yaml.  Fix ownership.
Merge pull request #130 from DataDog/agent_config_fix

Fix datastax agent configuration
Having the group declared twice is not idempotent and is redundant with the gid attribute of the user resource
Make su file update idempotent
Merge pull request #131 from benpillet/master

Having the group declared twice is not idempotent and is redundant with ...