generate-tools now merges metadata from different streams
Review fixes
Small tweak
Small tweak
Introduce constants
Merge pull request #1001 from wallyworld/simplestreams-merge

generate-tools now merges metadata from different streams

Fixes: https://bugs.launchpad.net/juju-core/+bug/1386509

juju metadata generate-tools creates simplestreams metadata for tools tarballs.
When run more than once, it now appends any new metadata for a new stream to the existing metadata.