When datasource returns 'none' as unit it should be possible to change unit in panel

[dimrozakis]

What Grafana version are you using?

5.3.0

What datasource are you using?

CloudWatch

What did you do?

Upgraded grafana to 5.3.0 and visited one of my dashboards displaying S3 metrics from CloudWatch. All units that were previously manually configured were automatically converted to 'none'. When I tried to set them manually again, they'd automatically revert back to 'none'.

What was the expected result?

Either properly autodiscover units, or allow me to manually set them.

More context

This was caused by #13571. See #13571 (comment).

The main issue here is that if the autodiscovered unit is 'none', it shouldn't be forced on the UI. Maybe the user knows better what the unit is, or maybe they prefer it as 'short' instead.

It'd also be worth investigating why S3 metric units are not being discovered.