Add Traffic Mirroring for Istio Service Mesh

Traffic mirroring is a pre-stage for canary deployments.  When mirroring
is enabled, at the beginning of a canary deployment traffic is mirrored
to the canary instead of shifted for one canary period.  The service
mesh should mirror by copying the request and sending one copy to the
primary and one copy to the canary; only the response from the primary
is sent to the user.  The response from the canary is only used for
collecting metrics.

Once the mirror period is over, the canary proceeds as usual, shifting
traffic from primary to canary until complete.

Added TestScheduler_Mirroring unit test.

pkg/apis/flagger/v1alpha3/types.go

@@ -111,6 +111,7 @@ type CanaryAnalysis struct {
 	Interval   string                           `json:"interval"`
 	Threshold  int                              `json:"threshold"`
 	MaxWeight  int                              `json:"maxWeight"`
+	Mirror     bool                             `json:"mirror,omitempty"`
 	StepWeight int                              `json:"stepWeight"`
 	Metrics    []CanaryMetric                   `json:"metrics"`
 	Webhooks   []CanaryWebhook                  `json:"webhooks,omitempty"`