Make it easier to update paas2-auth-sidecar image

We have many projects that use the paas2-auth-sidecar image. Whenever we update this image we need to update multiple projects to pick up the new version.

Add an environment variable with the current image. This will allow us use this tag in the various places where we use paas2-auth-sidecar, so that they get the latest image on deploy.

See:

• [platon/paas2-auth-sidecar#1] Make it easier to update paas2-auth-sidecar