ast_coredumper: Add ability to use directory other than /tmp

The OUTPUTDIR environment variable can now be set either in the
environment itself or in ast_debug_tools.conf.  If set, it's used
for all work products instead of /tmp.

Also added the --tarball-config option that includes the contents
of /etc/asterisk when either --tarball-coredumps or --tarball-results
are used.

Change-Id: I66b2553319df61caea5b313d084f51978f730b4c