remove all trailing whitespace from custom docker options config

[benjamin-smith]

Python's rstrip function calls were not stripping whitespace from the role's docker_opts option. This caused the created file at /etc/systemd/system/docker.service.d/env.conf to have an extra line, like so:

[Service]
Environment="DOCKER_OPTS=--graph=/data-01/.docker-data
"

The options were then not loaded properly by the Docker daemon.

This PR removes all whitespace from the end of the docker_opts setting, which removes the extra line (as well as any other whitespace, which should not be needed).

Local:
Python 2.7.12
ansible 2.0.1.0

Remote:
Ubuntu 16.04 on EC2
Linux ip-10-0-1-80 4.4.0-53-generic #74-Ubuntu SMP Fri Dec 2 15:59:10 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux