Upgrade to helm v2.8.0

Upgrade helm v2.8.0 (including the packages from the k8s.io/helm path)

Improve S3 policy security

optionally allow anonymous read for GET operations when basic auth is enabled resolves kubernetes-helm/chartmuseum#34

Add restricted IAM policy

Closes #31
Every API call is working with this policy.

Add code-of-conduct.md

Refer to kubernetes/community as authoritative source for code of conduct

Adds a health check endpoint, at the root address.

The health check endpoint is reachable even when basic HTTP authentication
is enabled.

The health check confirms that the backing store is responding, returning an error
response if it is not, else a successful response.

Add X-Request-Id header

New file pkg/chartmuseum/logging.go created to handle all logging

New Logger methods added (Debugc, Infoc, Warnc, Errorc) that take a
gin context as an argument, which will prepend log messages with the
request count, and add request id (reqID) to key-values

Only test storages that are configured in the env

One may not have access to all cloud storages at a time. Allow the
testing framework to run even if it is missing a storage.

In the acceptance test and in the testcloud make targets, Storage tests
will only run for the corresponding defined storage variable i.e.:

Amazon tests will run only if TEST_STORAGE_AMAZON_BUCKET is defined and
Google tests will run only if TEST_STORAGE_GOOGLE_BUCKET is defined.

The build machine should catch any test outliers, as it defines
correctly all the cloud test environment variables.

Fix index response times increasing with many uploads