perform variable substitutions on sessionless launches

test plan:
  - install a tool with variable substitutions
  - do a sessionless launch
  - it substitute substitute variables

fixes PLAT-695

Change-Id: Ia4f0c35caeec182dc29bf07df2d1a9db45a244aa
Reviewed-on: https://gerrit.instructure.com/43617
Reviewed-by: Nathan Mills <nathanm@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>