Adding alerts support for Rocket.Chat; closing Issue-#1091

[oussjarrousse]

This pull-request, adds the support for alerts for Rocket.Chat https://rocket.chat/ (an open source slack alternative)
Based on the code available in the code-base that supports Slack. It addresses issue-#1091 and hopefully closes it.

The change add two new config.json variables:

"rocketchat_alert": true,
"rocketchat_url": "the_url_to_the_rocket_chat_web_hook",

The pull-request adds the following configuration to the cli/setup/setup.go:

askConfirmation("Enable Rocket.Chat alerts?", false, &conf.RocketChatAlert)
if conf.RocketChatAlert {
	askValue("Rocket.Chat Webhook URL", "", &conf.RocketChatUrl)
}	

It adds the function sendRocketChatAlert() to services/tasks/alert.go which is (almost) identical to the existing sendSlackAlert(). sendRocketChatAlert() is called from services/tasks/TaskRunner.go SetStatus() in a similar way to all other alerts (email, telegram, and slack).

It also adds a template file /templates/rocketchat.tmpl that specifies the format of the json request.content expected by Rocket.Chat web hook.

Testing is still required.

[oussjarrousse]

I tried to debug the issue. But I am not familiar with the project enough to figure it out.
I would appreciate your input.
Thanks