[WIP] Feat: enable Ubuntu as container build host

[mbologna]

What does this PR change?

Enable Ubuntu as container build host

GUI diff

No difference.

• DONE

Documentation

• doc-susemanager PR or issue was created (GitHub automatic link expected below) TBD

• DONE

Test coverage

• Unit tests were added TBD

• Cucumber tests were added TBD

• DONE

Links

Fixes #397
Manager-4.0 TBD

• DONE

Changelogs

If you don't need a changelog check, please mark this checkbox:

• No changelog needed

If you uncheck the checkbox after the PR is created, you will need to re-run changelog_test (see below)

Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• Re-run test "changelog_test"
• Re-run test "backend_unittests_pgsql"
• Re-run test "java_lint_checkstyle"
• Re-run test "java_pgsql_tests"
• Re-run test "ruby_rubocop"
• Re-run test "schema_migration_test_oracle"
• Re-run test "schema_migration_test_pgsql"
• Re-run test "susemanager_unittests"
• Re-run test "javascript_lint"
• Re-run test "spacecmd_unittests" (Test skipped, there are no changes to test)

[mbologna]

Still WIP, deprioritized in favor of cluster management.

[juliogonzalez]

Dear developer:

A branch for SUSE Manager 4.1 is now created on the internal repository, and all package SPEC files in the master branch of this repository are now updated to be '4.2.0' version.

All developers, regardless of being SUSE employee or not, need to keep the following things in mind:

1. Please rebase this PR against masterbranch and fix any conflicts.
2. If this PR includes schema upgrades, change the directory for the migration scripts to the correct one. At this precise moment it is schema/spacewalk/upgrade/susemanager-schema-4.2.0-to-susemanager-schema-4.2.1 but it could be a different one if this PR is still open after Uyuni 2020.08 is released.

SUSE employees must also keep in mind:

1. If you merge the PR here, you most likely want to backport it to the new Manager-4.1 branch.
2. You will need to adapt the PR when backporting for at least the changelogs and schema migrations (if there are on your PR).

[Shirocco88]

Hint: container_build_host Entitlement for Debian-9/10 and Ubuntu-16/18/20 is already enabled;

see https://github.com/uyuni-project/uyuni/blob/master/schema/spacewalk/common/data/rhnServerServerGroupArchCompat.sql#L761
insert into rhnServerServerGroupArchCompat ( server_arch_id, server_group_type)
values (lookup_server_arch('amd64-debian-linux'),
lookup_sg_type('container_build_host'));

[mbologna]

Closing as stale; will be resumed in the near future.