deployment: support load balancing

[mdonadoni]

Reported as an issue when using MetalLB

Authentication fails when serving REANA behind a load balancer, in particular due to the fact that session identifiers are not stable.

See the following code that generates the session identifiers (source):

def _create_identifier():
    h = sha512()
    h.update(f"{request.remote_addr}|{request.user_agent.string}".encode())
    return h.hexdigest()

request.remote_addr changes based on which load balancer/server serves the request. It might even just be caused by a misconfiguration of ProxyFix.