Add Slack notification support in detector rules

[dichiarafrancesco]

As requested in https://github.com/Yelp/terraform-provider-signalform/issues/8 we're going to add support for slack notifications in our detectors.

[blkperl]

Tested manually and Slack integration is working with this patch applied.

[marcingumkowski]

Could you show the example of Slack notifications in TF? this notifications = ["Slack,credentialID,channel,"]` doesnt work

[blkperl]

@marcingumkowski like this (although it looks like you have that):

notifications = "${var.team_slack}"

variable "team_slack" {
  description = "Team SignalFX Slack integration"
  default     = ["Slack,credentialID,team-channel"]
}