Updates to Scaling documentation

The scaling guides have some old inaccuracies and readability issues left over from the conversion from wiki pages, some very dated compatibility information, and could use some information about new scalability features in 3.0.

 - index: Small tweak to verbiage about the scaling_multiple_masters document
 - inventory_service: Switch guidance over to the inventory_service terminus, see bug 10289
 - passenger: Largely a cleanup of old 0.24-specific information and overall simplification of the document.
 - scaling_multiple_masters: Update to include information about SRV record feature in 3.0.  Re-order sections to hopefully make the decision tree for deployment options clearer.  Correct or add additional detail to configuration examples.