Make Fluentd to GCP Logging image self-contained

The previous Dockerfile depended on a fetch from a remote file from the Signals team to help configure td-agent. I've removed this dependency to make the creation of this Dockerfile self contained. I can't yet commit all the files (the binary for fluentd, the .rd file etc.) because shortly these will be rolled into the main distribution so until this I shall check in just the Dockerfile and our project specific Fluentd config file. I've also changed the base image to google3/debian:wheezy which is smaller than ubuntu and perhaps more likely to be in use anyway on the target platform. I've also adjusted the Fluentd config to use a file_pos setting to deal with log rotation.
Put the pos_file in /var/log/td-agent/... so that it does not need to be root to write in /var/lib/docker/...