ability to (re-)trigger runboat when the build has varnished from runboat instance

[pniederlag]

On OCA/l10n-germany#247 we lost "the connection" to the runboat instance.

Having some /ocabot build or alike to retrigger runboat would be something that could have helped us here

[sbidoul]

There is a runboat API to trigger a build but it's not public out of fear of abuse.