exposed outlierDetection configuration through GTP

[shriramsharma]

Fixes #104
Extended the GTP API to expose outlierDetection configuration.

Example GTP Config:

apiVersion: admiral.io/v1alpha1
kind: GlobalTrafficPolicy
metadata:
  name: gtp-failover-config
  namespace: sample
  annotations:
    admiral.io/env: stage
  labels:
    identity: greeting
spec:
  policy:
    - dnsPrefix: default # default is a keyword, alternatively you can use `env` (ex: stage)
      lbType: 1 #0 represents TOPOLOGY, 1 represents FAILOVER
      target:
        - region: us-west-2
          weight: 80
        - region: us-east-2
          weight: 20
      outlier_detection:
        base_ejection_time: 180
        consecutive_gateway_errors: 1000
        interval: 60

[psikka1]

LGTM 👍