add a flag that instructs realservers to collect certain flags from the host node. This allows the realserver to run as a daemonset - in rkt, each node has a unique set of environment variables that are computed on systemd init via bash, which can't be implemented in a Kubernetes daemonset.

This change will allow users to set an env-mode flag as well as the location of the environment file that contains all the vars scraped up by rkt init, which requires a volume mount in the deployment