Use utils._tempdir() which in turn uses _signals.terminator() for
this purpose.

This also changes utils._tempdir() to use utils._force_rmtree()
so that it is safe to use for cleaning up the staging directory.

This patch fixes orphaned temporary directories being left behind
in the artifact cache directory at forceful termination time.

element.py: Cleanup temporary staging directories on termination

See merge request !1080

Tristan Van Berkom authored Jan 16, 2019 and Tristan Van Berkom committed Jan 16, 2019

We should only display commands in detail strings, not in the
message texts.

This also updates tests/integration/sandbox-bwrap.py to expect
the new message string which only contains the command exit status
and not the whole command itself, this does not alter the validity
of the text case which is checking that we can obtain the expected
return value.