Merge pull request #1068 from wallyworld/generate-metadata-delete-option

Add --clean option to tools metadata generation

Fixes: https://bugs.launchpad.net/bugs/1390250

Add a --clean option to "juju metadata generate-tools"

This will delete any exiting metadata for the specified stream, so that new metadata replaces instead of merging. Metadata for other streams is untouched.

Also, drive by fix for go vet error.