Allow -v option to change log level

Setting logging level to 'NOTSET' would tell logging module to inherit
logging level from handlers. Since the verbosity of handlers is
controlled by -v option, this allows sign_apex.py to give verbose output
if -v is specified.

Test: th
Bug: 307191956
Change-Id: I9c1db107cf4eae80ab570d892b0465b3902298fd