openshift.ks: Add set_conf

Add the set_conf function, which takes the filename of a configuration file, the name of a setting, and a value for that setting and adds the setting to the configuration file if it isn't already present with the given value.

Add the set_sysctl function, which wraps set_conf for /etc/sysctl.conf.

configure_sysctl_on_node: Use set_sysctl and improve comments.

install_rhc_pkg: Use set_conf to set libra_server.

set_mongodb: Use set_conf.

configure_datastore: Use set_mongodb to set bind_ip.