Add labs artifacts to .gitignore

The training-labs scripts create various artifacts that should not be
tracked by git. They are created in four directories which are added
to .gitignore by this changeset.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I89b103216ca3993448948966b832886bdd7a0ccf
Merge "Add labs artifacts to .gitignore"