FR: have LTM parse serial console of test VMs

[lrumancik]

Having LTM monitor the test VM's serial consoles will give us more information on the status of the VM.

We can monitor for PANIC/OOPs messages. This would allow us to catch hung tasks earlier instead of having to wait the full 1hr test timeout. Additionally, we probably want to reboot if we see these regardless as it may impact the integrity of subsequent tests.

Right now we update the test status via the test VMs GCE metadata. As you are not advised to update a VMs metadata super often, we limit the VM to updating its metadata every minute. Using the metadata as a means of communication between the test VM and LTM server can be inconvenient at times. If we monitor the serial console, we can get rid of using the GCE metadata for test statuses.

[lrumancik]

If we want to conditionally save the serial console, we can also use this to determine when we want to save it.