mgr/dashboard: clean-up python unit tests

mgr/dashboard: clean-up python unit tests

Fix pylint issues.

Refactor JwtManager check to avoid code duplication (found by pylint
when --jobs=1).

Fix issue with DashboardException.code property, using abs() on
potentially None attribute.

Disables Doctests in services/rbd.py that are actually integration tests
(they check the value of rbd.RBD_FEATURES_NAME_MAPPING). Ideally, these
kind of tests should be explicitly executed in an integration testing
stage, rather that unit-testing. Disabled tests have been prepended with
@DISABLEDOCTEST token.

Fixes: https://tracker.ceph.com/issues/40487
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>