Improve help strings

Make help strings consistent:
* Add missing spaces between words
* Capitalize first word of help
* Add "." at end of string
* Improve wording and capitalization

This follows the oslo.config style guide:
http://docs.openstack.org/developer/oslo.config/styleguide.html

Change-Id: I8243909249423b6b58ccda0d800856f46b0953c5
Merge "Improve help strings"