Add a configuration setting to the plugin for setting the percentage of users to send to the canary build.
The configuration for a plugin needs to be added in two places:
1) In `server/configuration.go` as part of the configuration struct
2) In `plugin.json` under "settings_schema" (this will add it to the system console)
see for more details https://developers.mattermost.com/extend/plugins/manifest-reference/
No testing required.