Slack VarOverride functionality doesn't work the slack url

[moadqassem]

Go version: go.11.1
OS: MacOS Mojave
Consul Version: 1.4.2
Consul-Alerts Version: 0.5.0

Description:

The main reason to use the VarOverrides is to have different notifiers paths. When I am using slack profile, the notifier variables are actually not overriden, the reason for that is, the url filed in the SlackNotifier is ignored. for example:

{
  "Interval": 10,
  "NotifList": {
    "slack":true
  },
  "VarOverrides": {
    "slack": {
      "url": "https://hooks.slack.com/xxx"
    }
  }
}```

[moadqassem]

Here is the PR which fixes the issue: #252