Merge lp://staging/~sinzui/juju-release-tools/many-indexes into lp://staging/juju-release-tools

This branch ensures there is always an index.json present in the streams we create.

Historic juju uses index.json. New juju will create index2.json. We need to ensure that every stream we create has an index.json. We need to preserve the current index.

The content rarely changed in the past. Every six months a new series was added, and some series like trusty got new archs. The date stamp is the only change that happened every time we ran generate-tools. the index files tells all the supported series and archs to read the released product file to find juju agents. The product file is the same for new juju.

I could have copied an existing index.json into the tree and wrote a bash script to change the date. I elected instead to create it with python so that we can ensure the content is always valid json (which generate-tools did). You can see an example of the out put at http://pastebin.ubuntu.com/8856043/ and compare it to the output of http://juju-dist.s3.amazonaws.com/testing/tools/streams/v1/index.json

I made a last minute decision to add vivid (15.04) to the list. We are a minimum of 3 weeks away from 1.21.0 juju getting into vivid. Parties that need to test vivid, like us, need vivid agents for 1.20.11. We need to create them before we setup of vivid-slave to start testing next week.