Adds worker shell handler with dummied build

shell_nobuild.Handler is identical to shell.Handler except
the function build doesn't call deployer or truly build
an image, instead proceeding directly from unittest to
counting the assembly as READY.

Change-Id: I1ee70c3f1886901f2b70f74cf027f5d62fc5a24b
Merge "Adds worker shell handler with dummied build"