Refine devices count dashboard panel

[aieri]

In #340 we discussed a few improvements over the plain removal of that panel:

I think it's ok and make sense to remove this panel, since it was misleading.

I have two questions:

• Should we fix also "Devices count" to count unique device, so sum should be by device (if my assumption that device means device id)?
• Can we do all number from first panel over time? What I mean with this, is that "Device Count" should provide the number of unique device for period window user is using.

Originally posted by @rgildein in #340 (review)

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The internal ticket has been created: https://warthogs.atlassian.net/browse/SOLENG-899.

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[Deezzir]

We should also switch from the instance label to something like juju_unit for alerts and the dashboard. The instance label is always set to localhost:10201, which is not very informative.