Merge lp://staging/~sinzui/juju-release-tools/publish-centos into lp://staging/juju-release-tools

Archive centos7 agents

This branch is a half step to support centos7 agents in streams. I agreed with Martin that we want to generalise win_agent_archive.py to be the single means to add, get and delete agents made by CI. This branch broadens the rules to allow me to place a centos agent into s3://juju-qa-data/win-agents/. The assemble-streams.bash script will find the new agent because the "get" command matches juju version, not OS name. Both CI and Jerff will find the new agent when the updated win_agent_archive.py is uploaded.

Tomorrow I rename both the script and the win-agent dir. This will require me to change several other things. I don't want to change them today as I release.