Fix installing latest git from PPA in bionic

[trxcllnt]

keys.openpgp.org strips user IDs from keys unless the key owner grants permission to share them. This leads to gpg rejecting the keys instead of importing them, and failing to install the git feature.

This PR enforces an explicit order when testing and importing keys from the keyservers list, ensuring keys.openpgp.org is the last keyserver to be tried. Previously, keyservers were selected based on bash's associative array key enumeration order, but this order is different between Bash v4 (in Ubuntu Bionic) and v5 (Ubuntu Focal+).

I believe the issue isn't seen in Bash v5 because the ubuntu keyservers are enumerated first, not because the gpg version in Focal accepts keys without user IDs.

If this is the case, we should either stop attempting to import keys from keys.openpgp.org, or encourage the owner of E1DD270288B4E6030699E45FA1715D88E1DF1F24 to grant openpgp.org permission to share their user ID.

Fixes #1055 and adds a new scenario test.

Note: I believe this fix may need to be applied to all features modified in #1016.

[trxcllnt]

It appears mirrorlist.centos.org no longer exists, and that's why the centos-7 scenario tests are failing.

[samruddhikhandale]

Thank you for the fix, appreciate it.

Let's merge once the tests are green!

[samruddhikhandale]

Note: I believe this fix may need to be applied to all features modified in #1016.

Most likely, yes!
Let's see how this PR works and we could add similar fix for others. Opened #1062 for tracking