worker/uniter: fix debug-hooks exit status output

[axw]

Description of change

Fix the debug-hooks bash scripts to return the
exit status correctly. We previously sent the
script to bash stdin, which meant a $ had to
be escaped; now it's written to disk, and does
not.

QA steps

1. juju bootstrap && juju deploy ubuntu
2. juju debug-hooks ubuntu/0
3. juju config ubuntu hostname=foo
4. (in debug-hooks) exit 0
   (check juju status output -- unit should be idle/not in error)
5. juju config ubuntu hostname=bar
6. (in debug hooks) exit 1
   (check juju status output -- unit should be in error)

Documentation changes

None.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1732233

[axw]

Exiting from a debug-hook session puts hook in error state

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju