Update istio/mixer Docker tag to v1.5.7

This MR contains the following updates:

Package	Type	Update	Change
istio/mixer	ironbank-docker	patch	1.5.6 -> 1.5.7
istio/mixer		patch	1.5.6 -> 1.5.7
istio/mixer	stage	patch	1.5.6 -> 1.5.7

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