Use generator to enable ostree-remount.service and ostree-finalize-staged.path

We struggled for a long time with enablement of our "internal units",
trying to follow the philosophy that units should only be enabled
by explicit preset.

See https://bugzilla.redhat.com/show_bug.cgi?id=1451458
and https://github.com/coreos/rpm-ostree/pull/1482
etc.

And I just saw chat (RH internal on a proprietary system sadly) where
someone hit `ostree-remount.service` not being enabled in CentOS8.

Thinking about this more, I realized we've shipped a systemd generator
for a long time and while its only role until now was to generate `var.mount`,
but by using it to force on our internal units, we don't require
people to deal with presets anymore.

Basically we're inverting things so that "if ostree= is on the kernel
cmdline, then enable our units" and not "enable our units, but have
them use ConditionKernelCmdline=ostree to skip".

Drop the weird gyrations we were doing around `ostree-finalize-staged.path`
too; forking `systemctl start` is just asking for bugs.

So after this, hopefully we won't ever again have to think about
distribution presets and our units.

tests/kolainst/destructive/staged-deploy.sh

@@ -8,6 +8,10 @@ prepare_tmpdir
 
 case "${AUTOPKGTEST_REBOOT_MARK:-}" in
   "")
+  # Test our generator
+  test -f /run/systemd/generator/multi-user.target.wants/ostree-finalize-staged.path
+  test -f /run/systemd/generator/local-fs.target.requires/ostree-remount.service
+
   # Initial cleanup to handle the cosa fast-build case
     ## TODO remove workaround for https://github.com/coreos/rpm-ostree/pull/2021
     mkdir -p /var/lib/rpm-ostree/history
@@ -19,7 +23,6 @@ case "${AUTOPKGTEST_REBOOT_MARK:-}" in
   # Start up path unit
     systemctl enable --now ostree-finalize-staged.path
   # Write staged-deploy commit
-    export OSTREE_SYSROOT_DEBUG="test-staged-path"
     cd /ostree/repo/tmp
     # https://github.com/ostreedev/ostree/issues/1569
     ostree checkout -H ${commit} t
@@ -29,8 +32,7 @@ case "${AUTOPKGTEST_REBOOT_MARK:-}" in
     systemctl show -p SubState ostree-finalize-staged.path | grep -q waiting
     systemctl show -p ActiveState ostree-finalize-staged.service | grep -q inactive
     systemctl show -p TriggeredBy ostree-finalize-staged.service | grep -q path
-    ostree admin deploy --stage staged-deploy | tee out.txt
-    assert_file_has_content out.txt 'test-staged-path: Not running'
+    ostree admin deploy --stage staged-deploy
     systemctl show -p SubState ostree-finalize-staged.path | grep running
     systemctl show -p ActiveState ostree-finalize-staged.service | grep active
     new_mtime=$(stat -c '%.Y' /sysroot/ostree/deploy)