Update 9000 mtu limit

[noahmehl]

For modern hardware and networking gear, MTU's above 9000 are common now. For instance, we have switches that support 9216, and intel 10gbit adapters that support 15500. There are a few reasons to want this:

• Being able to offer the default fabric networks to end users at 9000 (9216 would be plenty, as it should be 9000 + 50byte overhead)
• Being able to offer end users a flat VLAN for their use. Allowing them to utilize VXlans with a 9000 MTU

# dladm show-linkprop -p mtu ixgbe0 LINK PROPERTY PERM VALUE DEFAULT POSSIBLE ixgbe0 mtu rw 9000 1500 1500-15500

I am testing an update to the constants.js to provision compute nodes with an MTU of 9216. I will update with the results here.

I think bumping up the max to 9216 would suffice for our use case, but still provide the ability to keep users from setting it to something horribly detrimental.

[noahmehl]

Ok, so even though you can update this value, network/physical actually checks this as well, and fails :(

[noahmehl]

Alright, we finally got an update to platform-image that will allow up to 65535 MTU:

TritonDataCenter/smartos-live#860 (comment)

Do we want to update this now? And if so, to what? I'm wondering if we want to make it update-able via metadata in Triton?