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.
Typing make will just do "build" and I have deliberately made it so rather than adding a "all: build pus" rule because I think it is safer and more flexible to decouple the process of building from pushing. For example, to help me locally debug and test an image I want to build it, test it out, and when I am happy with it then I will push it. I don't expect anyone other than me is going to run this Makefile which is mainly designed to mitigate the joint effects of RSI and erratic typing on my part.