Fix RequiredOptError when missing lock_path

When lock_file is not present on the [oslo_concurrency] section of the
config file, we are getting a misleading error saying that it is not
preset on the DEFAULT group. This commit fixes it by providing the group
arg to the RequiredOptError constructor.

Closes-Bug: #1966320

Signed-off-by: Thiago Brito <thiago.brito@windriver.com>
Change-Id: Idee6987739917c7eae4faee15a66085a9ef7f6d2