Add support for threshold alert type in WaveFront

[pradeeppanga]

Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE REQUEST

What happened:

What you expected to happen:
Add support for threshold alert type in WaveFront

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• alert-manager version
• Kubernetes version :

$ kubectl version -o yaml

Other debugging information (if applicable):

- controller logs:

$ kubectl logs