If bootstrap fails, remove the virtualenv

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

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
Add retries to fetch_teuthology()

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