Adding basic implementation of lmctfy exec driver.

This adds a new execution driver for Docker which will use lmctfy. Docker can be
instructed to use lmctfy driver by passing the flag ‘-e lmctfy’ on startup of
the docker daemon.
This driver requires lmctfy version 0.4.5 or greater to be
installed. (https://github.com/google/lmctfy).

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan (vishnuk@google.com) (github: vishh)