JCLOUDS-753: Make ConnectionSpec configurable in the OkHttp driver

Makes the ConnectionSpecs injectable so providers and users can provide a custom one.
This should also fix [JCLOUDS-753](https://issues.apache.org/jira/browse/JCLOUDS-753), as the change provides a clean way of setting the supported TLS protocols.

/cc @adriancole @andreaturli 
Oh, that makes sense. This approach is already used in Chef to [bind Ohai info](https://github.com/jclouds/jclouds/blob/master/apis/chef/src/main/java/org/jclouds/ohai/config/OhaiModule.java#L80-L88) and makes perfect sense to use it here too. Thanks for the suggestion!