Docs (master): Improved quickstart brooklyn.properties

Same as 0.5 Docs PR https://github.com/brooklyncentral/brooklyn/pull/712:

Significantly improved brooklyn.properties.
* File broken into sections.
* Welcome message added.
* Quickstart Options (AWS, Rackspace and Geoscaling credentials) pulled to the top of the file.
* Sensible defaults selected for Getting Started process.
* - Ubuntu 12.04 LTS AMIs specified for all regions 
* - m1.smalls by default.
* All jclouds locations spelt out as named locations, then commented out.