Remove unnecessary check for writable output dir.

os-config-applier is useful by non-root users who have access to
write the files they need to in sub-directories under the main output
directory.

Change-Id: I52af0bd076f3fef99ea09a3074f0b0d5b7e9afca
Make main() return rather than call sys.exit

Fixes tests not all being run and also makes main() more directly
testable. Also adding tests for log messages when main() is called.
Finally adding argparse to requirements.txt to fix python 2.6

Change-Id: I7f152c9aa1e99d0fd3d763871c96cf60840e3ff7
Merge "Make main() return rather than call sys.exit"