Upgrade rubocop

Rubocop 0.18.1 has been released, which includes the reported
inconsistent class documentation cop triggering fix. This release also
contains some new cops and fixes others, triggering new violations on
existing rubocopped code.

- Upgrade rubocop to 0.18.1
- Fix new/corrected violations

Change-Id: Ia66ce8521c06a94bc28928784161d657da50f215
Addresses: blueprint rubocop-for-object-storage