With its 'append' action, optparse appends values to the supplied
default value. This leaves the user with no way of overriding the
default value. If you want to support overriding, you need to manually
handle setting the default value after parsing.
gflags supports overriding the default value so we must do the same.
Change-Id: I5810e16806e74876c7a310edbb6f8f27f0a3313b
d089fb642a