Start modifications to Riff-Raff to lookup instances via Prism

 - Introduce a new interface called 'Lookup' that abstracts the lookup away from
   any one source of information
 - Implement Lookup using the existing DeployInfo data source
 - Add LookupSelector that can select the underlying data source
 - Start migrating code over to use the Lookup mechanism
Further modifications to Riff-Raff to lookup instances via Prism

 - Change code in magenta and riff-raff to use Lookup instead of DeployInfo
 - Attempt to make the credential builder more sane for this world (and work in the CLI)
Implement Lookup interface powered by Prism

 - make calls to prism to fulfil information requests
 - add prismUrl configuration option
 - also converted a bunch of List uses into Seq
Add validation of DeployInfo vs Prism lookup

 - have a switch that enables validation of lookup responses and logs results
Update release notes
Move location of transposition code and use in deployinfo template
Add new properties to documentation
Update test to use new transposeBy method
Implement a retry for calls to prism
Merge pull request #136 from guardian/sh-decouple-lookup

Decouple lookup and add support for Prism