Allow SSLContext instance to be reused between env

[Jason Gerard DeRose]

When Dmedia is syncing between different local devices, the SSLContext instance could be the same, and only the env['url'] would be different. As creating the SSLContext is rather expensive, it would be nice to be able to reuse it.

So I'm extending env['ssl'] to support a 'context' item, which when present should be the SSLContext instance.

Currently you can do this:

env = {
    'url': 'https://192.168.0.3:6984/',
    'ssl': {
        'ca_file': '/my/private/authority.ca',
       'key_file': '/my/client.key',
       'cert_file': '/my/client.cert',
    }
}

And I'm proposing a change that will likewise let you do this:

env = {
    'url': 'https://192.168.0.3:6984/',
    'ssl': {
        'context': <SSLContext instance>,
    }
}