Provisioning CGP endpoints with 1500 MTU is broken

[jonstout]

By default the web service assumes GCP endpoints use an MTU of 1440. Because MTU isn't explicitly set on endpoints in the syncer, the initial sync reverts any 1500 MTU endpoints to 1440. Also because the field that sets the MTU is only set upon endpoint selection in the UI, edited connections revert the MTU to 1440 on GCP endpoints. Manual MTU updates made in the database don't appear to be caught by the diff routine; This needs to be fixed as well.