Add  docstrings for utils.helpers
Add  docstrings for utils.helpers

Cleaning up a little technical debt by adding docstrings
for our public APIs.

Note: I plan to submit a patch per file as part of this bug work.
These patches won't depend on each other. The last patch of
the series will close the bug.

Change-Id: I11c23e3be430daa3e618904db4b4016105c7fc04
Partial-Bug: #1614594

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib