Cache Certificates in SecurityCreds

Instead of loading the files when the SSL connection is initiated, cache the credentials in the `SecurityCreds` object to speed up integration tests.  Also convert the class to work more like a dictionary. 
:shrug: Early on you suggested a dictionary, but I thought I'd borrow some `dict` behavior without actually deriving from `dict`.  Properties usage would not be much different than before except I guess I could do the same automatic file caching.

The initial motivation for the switch was to reduce code duplication.

Quite happy to redo as properties.  Should be pretty easy now that the functionality is working.