gen_template_docs: only generate defined sections
gen_template_docs: only generate defined sections

Add additional checks in order to not write out documentation sections
for things not present in the input templates.

This removes empty 'Service Accounts' and 'Volumes' sections of the
documents.

knrc/application-templates,josefkarasek/application-templates,kyguy/application-templates,errantepiphany/application-templates,jboss-openshift/application-templates,bdecoste/application-templates,rcernich/application-templates,bparees/application-templates,douglaspalmer/application-templates