[testgrid] create a tgrun kurl addon

[marccampbell]

Currently we run the tgrun run command manually. Instead, we should include it as an internal addon in kURL that we can use.

This would allow us to create new instances on packet to execute testinstances without any manual steps involved.

This would be a simply deployment that has tgrun run with an optionally configurable testgrid-api endpoint address that can be defined in the kURL spec.

[markpundsack]

@marccampbell Do you still want to do this? Seems like there wouldn't be a lot of value in a tgrun kurl add-on outside of our internal testing.