Allow target exercises to run independently

Both target and base exercises will run according to the value
set by RUN_EXERCISES. For the gate job, we need the target
exercises to run out of the grenade.sh script.
This patch also defaults the smoke tests to False. They
will also be triggered by the gate scripts.

Blueprint nightly-upgrade-testing

Change-Id: I325d025cec6bacfa2bd7f3b48bed4a4abdacef76