Adds gnome-disks to sd-devices tests

[emkll]

Status

Ready for review

Description of Changes

Fixes #511. This PR is mostly to help with review and testing of
freedomofpress/securedrop-export#63 and freedomofpress/securedrop-builder#158

Changes proposed in this pull request:
Adds gnome-disks to sd-devices VMs

Testing

• Check out this branch of securedrop-workstation
• Check out branches in securedrop-export 0.2.3 securedrop-export#63 and Adds gnome-disk-utility to dependencies of securedrop-export package securedrop-builder#158
• Build securedrop-export 0.2.3 using those branches
• Copy securedrop-export 0.2.3 to sd-workstation folder of this repo
• Run make clone
• make all is successful
• make test has all tests passing (specifically test_sd_export_package_installed)
• Running gnome-disks inside sd-devices opens the gnome disk utility

Merging

The following tasks should all be done in one pass, after all 3 PRs have been reviewed/approved:

• Drop the temporary commit
• Merge securedrop-export 0.2.3 securedrop-export#63
• Create 0.2.3 tag in securedrop-export
• Re-run CI on Adds gnome-disk-utility to dependencies of securedrop-export package securedrop-builder#158
• Merge Adds gnome-disk-utility to dependencies of securedrop-export package securedrop-builder#158
• Merge this PR

Checklist

If you have made code changes

• Linter (make flake8) passes in the development environment (this box may
  be left unchecked, as flake8 also runs in CI)

If you have made changes to the provisioning logic

• All tests (make test) pass in dom0 of a Qubes install

[conorsch]

Local testing shows no problems. I'm going to proceed with dropping the temporary commit, then merging the corresponding PRs in other repos. Small change to the "Merging" instructions: I'm not going to proceed with tag generation. After merging, nightly builds will be added to the test repo, then on Monday we can proceed with final prod package prep. (Already discussed this small change to plan with @emkll out of band, summarizing here for visibility.)

[conorsch]

After preparing local packages as described, test plan checks out:

• Run make clone
• make all is successful
• make test has all tests passing (specifically test_sd_export_package_installed)
• Running gnome-disks inside sd-devices opens the gnome disk utility