Fix realhw functests

[aieri]

This commit attempts at fixing "realhw" functional tests, specifically for the following scenario:

tox -e func -- -v --realhw --model test --collectors ipmi_dcmi ipmi_sel ipmi_sensor --base [email protected] --nvidia --keep-models

List of changes:

• Move rendering the bundle to a fixture and disable redfish if the collector isn't listed - this avoids tests complaining about incorrect redfish credentials when you're not interested in testing redfish anyway
• Update README to consider the work we did in dev-environment.md to be able to run tests on a single machine
• Fix incorrect hardware-exporter config search path

[aieri]

Sorry I ran out of time so this remains a work in progress if someone wants to continue this work. test_cos_agent_relation fails for me for reasons I don't understand.