Adds an option to create configdocs as an empty colleciton. This is done
as an explicit flag (--empty-collection) on the command as opposed to
using empty files to prevent accidental emtpy file loads leading to
frustration.
Since this introduced a new flag value for the CLI, the CLIs using flag
values were updated to use the standard is_flag=True instead of the
flag_value=True or some other value when a boolean flag is expected.
Minor updates to CLI tests due to moving to responses 0.10.2
Depends-On: https://review.openstack.org/#/c/614421/
Change-Id: I489b0e1183335cbfbaa2014c1458a84dadf6bb0b
667a538330