https://github.com/openshift/router/pull/291 gave the router
the ability to use stats credentials contained in separate username
and password files for security reasons.
This commit modifies the Ingress deployment so that the router-stats
secret is mounted into router pods, and the filenames for the username
and password files are passed to the Router binary via new
environment variables.

pkg/operator/controller/ingress/deployment.go:

Add a new volume and volume mount for the router stats secret.
Replace the STATS_USERNAME and STATS_PASSWORD environment variables with
the new STATS_USER_NAME_FILE and STATS_PASSWORD_FILE environment
variables.

pkg/operator/controller/ingress/deployment_test.go:

Update `TestDeploymentConfigChanged` to reflect the new stats-auth
volume.

Update `TestDesiredRouterDeployment` to verify that the expected
volumes exist with the proper secret references.

This commit is in support of Bug 1955822.