* Add and test activate function to acme.
This function can be used to check if our optional dependencies are
available and they meet our version requirements.
* use activate in dns_resolver
* use activate in dns_available() in challenges_test
* Use activate in dns_resolver_test
* Use activate in certbot.plugins.util_test
* Use acme.util.activate for psutil
* Better testing and handling of missing deps
* Factored out *_available() code into a common function
* Delayed exception caused from using acme.dns_resolver without
dnspython until the function is called. This makes both
production and testing code simpler.
* Make a common subclass for already_listening tests
* Simplify mocking of USE_PSUTIL in tests
