Add simplestreams metadata validation tool
Rename validate command
Code review tweaks
Minor tweaks to release tarball process:

1) ensure that juju-core_1.11.4.tar.gz is not include in itself

2) drop leading ./ from paths as this was confusing packaging tooling.
Rework implementation as per code review discussion
Merge trunk
Code review tweaks
Fix test failures
Merge trunk
Tweak variable names
Fix tests
Make the simplestreams metadata commands plugins
Add support for supert command --description
Correctly handle arguments for built in plugins
Add plugins tests
format
Add help support for plugins
Tweak arg handling for plugins
Merge trunk and resolve conflicts
remove double dash from generated container name