CertificateVerificationHTTPSHandler.https_open() rebuilt self._context

from the Certificate Manager on every request, throwing away any context the

caller passed to the constructor.
This broke the legacy ssl_cert fallback for hosting service accounts. When

a stored certificate does not match the server's hostname,

_build_ssl_context_from_ssl_cert() deliberately returns a context with that

certificate loaded and check_hostname turned off. That context was replaced

with a strict one that had neither, so those connections would fail

verification.
The handler now records whether the caller supplied a context, and rebuilds

the context only when it did not. Contexts that the handler built itself from

hostname and port are still rebuilt from the request URL, so a redirect to

another server continues to use the right certificates.

Added a test covering an explicit context surviving https_open(), which

  failed before this change.
Added a test covering a context built from the hostname and port

  arguments.
Ran unit tests for reviewboard.certs and reviewboard.hostingsvcs.