Update changelog.md with 1.21.1 release notes.
Update changelog.md with 1.21.1 release notes.

telstra/open-kilda,telstra/open-kilda,jonvestal/open-kilda,telstra/open-kilda,jonvestal/open-kilda,jonvestal/open-kilda,jonvestal/open-kilda,telstra/open-kilda,telstra/open-kilda,telstra/open-kilda,telstra/open-kilda,jonvestal/open-kilda