Pass along Env Vars at Narrative startup, fix problematic defaults for docker config

[sychan]

These commits modify the Lua code to look for an environment variable called NARR_ENV_VARS which is then inserted into the default docker client config for environment variables passed to along at narrative container startup.

The value for NARR_ENV_VARS should an array of NAME=VALUE strings. For example, the following docker command will startup the nginx container and set the CONFIG_ENV variable to appdev for all narratives started by the Docker.lua module:
docker run -it -v /var/run/docker.sock:/var/run/docker.sock -e 'NARR_ENV_VARS=["CONFIG_ENV=appdev"]' kbase/nginx:latest

This commit also fixes the default docker configs for CMD and USER so that they don't clobber the settings built into the narrative image. The old defaults set CMD to /bin/bash, which stopped the narrative from starting up, and set the running user to nobody, making it impossible to update the config.json file

[coveralls]

Coverage remained the same at 13.123% when pulling 2564b33 on dockerize into 47e748b on develop.