[aws|compute] Updates 'connection' references

`connection` is being deprecated within Fog::Model because it actually
refers to the Fog::Service not the Fog::Connection.

This replaces references to the instance variable to use the accessor so
will continue to work following deprecation.
[libvirt|compute] Updates 'connection' references

`connection` is being deprecated within Fog::Model because it actually
refers to the Fog::Service not the Fog::Connection.

This replaces references to the instance variable to use the accessor so
will continue to work following deprecation.
[rackspace|storage] Updates 'connection' references

`connection` is being deprecated within Fog::Model because it actually
refers to the Fog::Service not the Fog::Connection.

This replaces references to the instance variable to use the accessor so
will continue to work following deprecation.
[core] Deprecates 'connection' accessor

The 'connection' accessor in collections and models actually refered to
a subclassed instance of Fog::Service which creates confusion in the
code.

References to 'connection' could have meant the service or the
Fog::Connection held within that service.

This deprecates the connection accessor and replaces it with `#service`
as a read only value. When a collection or model is initalised then
service should be passed.

This commit also updates all the changes to @connection made by
providers in model initialisers since these depending on the presence of
the 'connection' key. The key is still accepted by outputs a warning.
[atmos] Updates reference to service
[aws|auto_scaling] Updates reference to service
[aws|beanstalk] Updates reference to service
[aws|cdn] Updates reference to service
[aws|cloud_watch] Updates reference to service
[aws|compute] Updates reference to service
[aws|dns] Updates reference to service
[aws|elasticache] Updates reference to service
[aws|elb] Updates reference to service
[aws|glacier] Updates reference to service
[aws|iam] Updates reference to service
[aws|rds] Updates reference to service
[aws|storage] Updates reference to service
[bluebox] Updates reference to service
[Brightbox] Updates reference to service
[clodo] Updates reference to service