Utilization: Add Disk Usage chart

[mareklibra]

and refactor prometheus queries for disk capacity so they
can be reused for utilization.

[coveralls]

Pull Request Test Coverage Report for Build 1310

• 11 of 11 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 87.331%

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Totals
Change from base Build 1297:	0.03%
Covered Lines:	3193
Relevant Lines:	3507

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💛 - Coveralls

[mareklibra]

In the memory/storage utilization charts, the total is used as maximum for Y-axis.

If total is much higher than actual consumption, the chart does not show "progress" vividly as the consumption is nearly to be a constant.

What about using

• maximal historic value (from the given range) as Y-axis maximum
• but units (KiB, TiB, etc) from the total?

What others think?

[rawagner]

I would vote for using value AND units from max historic value

[mareklibra]

And yet another (better) possibility handling the case with "almost constant" values within the range:
For max-y, use max historic value multiplied by 2 or cluster's capacity total, whatever is lower.

This approach handles:

• near-constant values
• values << total

[atiratree]

+1