Remove exercises tests from grenade runs

For now on we're going to use smoke + scenarios tests.

Only devstack still uses exercises, so many references to it, and to exercises
to be skipped were kept.
Depends on: Iacb60ab011d2c7d90885b339b3c8454bd3de8219

Change-Id: I76305fae46a662177c6edcc9eca237c1d64be137
Closes-bug: #1023131
Merge "Remove exercises tests from grenade runs"