Clarified directives in Neutron api-paste.ini

Clarified directives in Neutron api-paste.ini for controller,
network, and compute nodes based on a working configuration. Also
cleaned up some formatting issues.

Closes-Bug: #1250583
Change-Id: I2dd2279f6d69c0082cdb1390ea15f72bc3a83bea
(cherry picked from commit f5b214adc072cd46e6415db1958417cae29ea33b)
Merge "Clarified directives in Neutron api-paste.ini" into stable/havana