[1.8.x] Fixed #24769 -- Cast optparse verbosity argument to an integer for better backwards compatibility.

Using `BaseCommand.options_list` makes Django use the legacy optparse
parser, which does not set the verbosity attribute correctly. Now the
verbosity argument is always cast to int. Regression in 85686386 (#19973).

Initial report and patch from blueyed.

Backport of a0047c62 from master