pkgrepo: fix enabling/disabling repositories on Debian

[marmarek]

What does this PR do?

Fix enabling/disabling repositories on Debian.
On plain Debian, enabled/disabled argument was ignored.

What issues does this PR fix or reference?

Fixes #27067

Previous Behavior

Adding/enabling Debian repository with pkgrepo.managed state did nothing if the repository was already present but commented out.

New Behavior

Repository correctly enabled.

Tests written?

Yes

Commits signed with GPG?

Yes

[dwoz]

@marmarek Since this is a bug, can you add a regression test?

[marmarek]

Added.

[marmarek]

When forward merging, salt.utils.is_true needs to be changed to salt.utils.data.is_true.

[dwoz]

@marmarek We are no longer accepting PRs against 2017.7, please re-open this against master if it's still an issue. You can reference this PR in the new one so we can retain the history.

[marmarek]

@marmarek We are no longer accepting PRs against 2017.7, please re-open this against master if it's still an issue. You can reference this PR in the new one so we can retain the history.

Have anything changed regarding a policy for bugfix pull requests? Last time they should be made against oldest supported branch, not master.
Anyway, sure, I can rebase it on a newer branch, but I'd like to know what I need to do, to have it merged - this one was already waiting half a year with an approving review and without any comment what's wrong.