Not all my_activity.py users have accounts on all servers the utility
queries. This results in access errors reported when the utility is
ran.

Also, many users never upload patches on some servers even though the
account is there, there is no point in querying those servers.

This patch allows to pass in a list of servers to exclude from the
list of queried servers.

BUG=none
TEST=invoked the utility as follows:

  $ my_activity.py --skip_servers \
    pdfium-review.googlesource.com,skia-review.googlesource.com

  observed that the 'ERROR:root:Access Errors:' messages are not
  generated any more.

Change-Id: I7d676e45ee99d883a153b58acb778a2e2d5ed74a
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1487342Reviewed-by: Aaron Gable <agable@chromium.org>