Minor update changelog.md with 1.21.0 release details.
Minor update changelog.md with 1.21.0 release details.

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