Should use a secret for sensitive brigade-github-app configuration

[krancour]

Currently, the brigade-github-app chart creates a config map that includes a sensitive key.pem field. The deployment mounts this config map to /etc/brigade-github-app and, in turn, the containerized brigade-github-app process in each pod consumes /etc/brigade-github-app/key.pem.

Since the contents of key.pem are sensitive, it would be more proper and more safe to store this in a secret.

This can be accomplished pretty easily with changes to the chart and no changes to the brigade-github-app software itself.

This is not a breaking change:

1. Eliminate the brigade-github-app config map (since key.pem) is its only field anyway.

2. Add key.pem field to the existing brigade-github-app secret.

3. In the deployment, mount the brigade-github-app secret to /etc/brigade-github-app in place of the brigade-github-app config map.