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 
Recalling @adriancole's recent comments on optional injection, is there any way we can get rid of this?