Now that we advertise MTU via DHCP and RA by default, there is no reason
to keep the option available for configuration. Other agents/plugins are
also encouraged to advertise MTU values to instances by their own means.
DocImpact: mark the advertise_mtu option as deprecated as of Newton.
Closes-Bug: 1576000
Change-Id: Ibf7d60dfc57bec090f16d909c050c09e7cfd9352