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.
Some background on this CL. The previous CL was written from scratch by me. Recently, the Signals team produced a new package for installing Fluentd and the configuration required to get it logging to Cloud Logging. Rather than taking an external dependency on a gs://signals/... location in this CL I have effectively in-lined their installation package. Consequently, some of the lines have changed e.g. the previous sed line where was written by me. The new sed line comes from inlining the Signals install. I'd like to follow the inline approach because then it makes easier to track any upstream changes in the Signals distribution and fold them in. I think it is important to inline the new install to make this image self-contained and not at risk of breaking due to a changed in gs://signals/...
I hope this makes sense.