properly set MTU in network interface configuration files for RHEL-based distributions.

Currently, the rh_ip.py execution module doesn't set network interface MTU correctly in the interface's rendered config file. According to RHEL documentation, this should be set as a separate shell variable in the config; currently Salt appends the MTU to the ETHTOOL_OPTS variable. I addressed this issue by updating the Jinja template that the module uses to render the config file, and by modifying the module itself to pass the MTU data into the template.