Allow disabling of ISpyB authentication (for dev stacks)

[phraenquex]

To fix the problem of the dev stgacks needing Frank's credentials, which is unnecessary security risk

[alanbchristie]

The existing code 'expects' ISPyB credentials (set via stack/Pod environment variables) and can't tolerate them not being set.

To avoid the error: -

1. A user can set appropriate ISPyB credentials in their stack, or...
2. We need to introduce new backend behaviour to tolerate (and possibly issue a log warning) if ISPyB credentials are not set.

[phraenquex]

@tdudgeon says there remains a bug (from few weeks ago) - @alanbchristie to review

[alanbchristie]

See also xchem/fragalysis-backend#332

[phraenquex]

@boriskovar-m2ms you need to test this - by doing something in a dev stack that an ISpyB-logged-in user is able to do. I.e. Boris should see no errors.