gce-xfstests to use custom FSTESTSETS (and LTM)

[tytso]

gce-xfstests passes the FSTESTSET to the kexec'd kernel using the kernel command line. The issue with passing this information here is that an extensive custom set of tests that is separate from the built-in groups of xfstests will not fit (e.g. generic/001 generic/002 generic/003 generic/004 ...etc)
Directly modifying the groups files in xfstests is a workaround, but this involves re-creating the xfstests tarball.

This change is to allow custom FSTESTSETS to be created and passed to the kexec'd image, without specifying the entire test set on the kernel command line.
The LTM should also be made aware of custom FSTESTSET files, and be able to pass the custom FSTESTSET from its JSON arguments into all of its child test appliances.

Details:
This may involve uploading a custom fstestset file or passing the custom fstestset into the gce instance metadata from a file, downloading the custom set into a file on the test appliance, kexec'ing, and having the kexec'd test appliance be aware of the custom set file rather than exclusively pulling the test set info from the kernel command line.