Improve OK_PKG_STRINGS

[Brian Murray]

In `autopkgtest-cloud/worker/worker` there is a set of TEMPORARY_TEST_FAIL_STRINGS for which there is then an exemption set - OK_PKG_STRINGS. This ends up being overly general though. The code contains the following:

 823 contents = log_contents(out_dir)
 824 temp_fails = [s for s in (set(TEMPORARY_TEST_FAIL_STRINGS)
 825 - set(getglob(OK_PKG_STRINGS, pkgname, [])))
 826 if s in contents]

I recently added dbconfig-common to OK_PKG_STRINGS (`'dbconfig-common': ['Temporary failure in name resolution']`) but the log line I really wanted to match on was more complex:

psql: error: could not translate host name "someserver" to address: Temporary failure in name resolution

Because there could still be a 'Temporary failure in name resolution' when connecting to the archive and that should be considered a temp fail but a problem connecting to the local postrgresql server should be considered a genuine failure.