Health checks variables and update golang and docker-distribution

[jriguera]

• Make redis health check fail not fatal. When redis is enabled for caching, the template automatically creates a health check for redis, but that behavior is not always desirable. I have checked when redis cache is down, registry can still work (with worse performance) but if the health check is enabled, registry will refuse to work and will show an error. This patch adds two new variables: "registry.health_storagedriver_enabled" and "registry.health_cache_enabled" with "true" as default values to maintain the compatibility.
• Update registry to 2.5.1 (because of the improvements in the catalog to list the containers)
• Update golang to 1.7.3

[jhunt]

Can you provide a description for this PR?

[jriguera]

Sorry, I got distracted. I will update the description.

[jhunt]

Thanks!