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.
Sorry, I've added that line back. Personally, I think it is fine to run the log gathering agent as root. That's exactly what would have happened if we had invoked the Signals Fluentd install by gs://signals/.... | sh
It also seems quite idiomatic to run td-agent / google-fluentd as root based on what I have seen of other configurations. If we made Docker produce log files with some other user and group -- what would it be? We could use td-agent or google-fluentd as the user and group but perhaps that would not suit the needs of other programs that need to also access the log files. Perhaps keeping the log files with root permissions is the least worst common denominator choice.