[Proposal] New setting to not reuse LVM volume groups

[ancorgs]

Problem

For historical reasons, YaST tries to reuse existing LVM volume groups when making a proposal.

That behavior can be very confusing in many situations. Some of the associated problems and historical factors are described here:
https://trello.com/c/Ib7QC298/19-proposal-strategy-for-reusing-stuff-in-proposal-bug1091998-and-others

Now Y2Storage::GuidedProposal is also used by D-Installer and we don't want to follow the YaST approach for reusing stuff. Since D-Installer represents a new playground, we don't need to play by the old rules and expectations.

Solution

Introduce a new setting into Y2Storage::ProposalSettings that will be used only by D-Installer (for the time being) and that will inhibit the mechanism to reuse existing LVM volume groups.

Testing

Added unit tests. Backwards compatibility ensured because all the previous tests keep working with no modification.

Note about the version

Version of the package not bumped because we are not in a hurry to submit this to Factory and @jreidinger is already working on another change for yast-storage-ng.

[coveralls]

Coverage increased (+0.0002%) to 97.754% when pulling 79a4eff on ancorgs:lvm_vg_reuse into 64989e4 on yast:master.

[imobachgs]

LGTM. There is no need to bump the version but we need to make sure that the change is available in the YaST:Head:D-Installer project too (it is linked, so it should be available).

[yast-bot]

✔️ Internal Jenkins job #1135 successfully finished

[yast-bot]

✔️ Public Jenkins job #449 successfully finished