Default to localhost bind and move var
The pulp_content_host was being defined in pulp-content, but being used
in the 'pulp' role. This moves the setting to the pulp/README.md and
removes it from being a default specified by main.yml.
Also the pulp_content_bind is now 127.0.0.1 instead of 0.0.0.0 as part
of story 4963.