Add support for generating LAVA test definitions

* Add LAVA package to provide primitives for converting Spread project
  actions and task definitions to LAVA test definitions
* Add -lava-testdef flag for generating LAVA test definition

Signed-off-by: Chase Qi <chase.qi@linaro.org>
3 jobs for spread-lava in 22 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #30120
format

00:00:15

failed #30121
test

00:00:07

 
  Build
skipped #30122
build
 
Name Stage Failure
failed
test Test
	go get github.com/snapcore/spread/spread
spread/google.go:19:2: missing go.sum entry for module providing package golang.org/x/oauth2/jwt (imported by github.com/snapcore/spread/spread); to add:
go get github.com/snapcore/spread/spread
spread/linode.go:22:2: missing go.sum entry for module providing package gopkg.in/tomb.v2 (imported by github.com/snapcore/spread/spread); to add:
go get github.com/snapcore/spread/spread
spread/lxd.go:7:2: missing go.sum entry for module providing package gopkg.in/yaml.v2 (imported by github.com/snapcore/spread/lava); to add:
go get github.com/snapcore/spread/lava
Cleaning up file based variables
ERROR: Job failed: exit code 1