[AWS/elasticache] Added mocking for parameter groups
[AWS/elasticache] Minor fix from RDS copy/pasta

Use Fog::AWS::Elasticache::NotFound instead of RDS
[Brightbox] Replace use of Fog::Connection

`Fog::Connection` is a wrapper around XML parsing behaviour which is no
longer in `fog-core` but will be moved to `fog-xml`

So should be using `Fog::Core::Connection` for the simpler behaviour.
Merge pull request #2666 from brightbox/update_core_connection

[Brightbox] Replace use of Fog::Connection
Use fog-json

This extracts the JSON encoding/decoding wrappers around MultiJson to a
separate gem.

Providers can use this shared behaviour in their standalone modules as
we work through things.
Merge pull request #2665 from sugarb/elasticsearch_parameter_group_mocks

[AWS/elasticache] Added mocking for parameter groups
Merge pull request #2667 from fog/use_fog_json

Use fog-json