Make flower-server start server with HTTPS by default

[panh99]

• Added --insecure argument to start server without HTTPS.
• Added --certificates argument to accept paths to (CA certificate, server certificate, and server private key)
• Added certificates argument to some functions to facilitate the above changes.
• Added E2E test bare-https to test starting server with SSL-enabled.
• Added necessary certificates under e2e/certificates/ for testing purposes.
• Amended e2e/test.sh and e2e/test_driver.sh.

Note

• flower-server command will fail without either --insecure or --certificates a b c.
• When --insecure is enabled, --certificates will be ignored. (This is mentioned in the help message.)
• e2e/bare-https/simulation.py is empty because simulation does not require any certificate.
• REST server part remains untouched b/c it's different from grpc-rere and grpc-bidi and I am not sure how to unify the three.

[danieljanes]

PR looks good!

run_driver_api and run_fleet_api are the targets for two additional commands that allow users to start the Driver API and the Fleet API independent of each other (see pyproject.toml:

Those two commands will have to support --insecure and --certificates, too. The easiest way to do it is probably through _add_args_common.

[tanertopal]

I would not commit these here but rather generate them each time the test runs and delete them afterwards. It's often easier to commit them, but at the same time, I would prefer to avoid having any secret keys or certificates even for testing committed to prevent any possibility someone might accidentally use them somewhere.

[danieljanes]

@tanertopal wdyt, should we also move the certificates directory into the bare-https dir? They are only required for that single test

[tanertopal]

If they are only required here, then let's move it. The directory could also contain the script to generate the certificates.