Add a couple unit tests for teuthology.parallel

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add a unit test simulating simultaneous checkouts

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Allow testing with online repos

It is not the default.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add another unit test

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Optionally, don't remove a repo when errors occur

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Don't remove the teuthology repo if errors occur

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>