Use python3 for CSITs

Use python3 for CSITs

- use python3 and pip3 in scripts
- update robot-framework-http library, so it can use python3
- update selenium2library, so it can use python3

Fixed merge conflict from cherry pick:
- prepare-csit.sh

Changed to github instead of gerrit-mirror, as the mirror is flakey and
broke one of the CSIT verify jobs.

Issue-ID: POLICY-3092
Signed-off-by: arkadiusz.adamski <aadamski@est.tech>
Change-Id: I5a6a4fe43e54366859049e1d43bbce599dacd14e
(cherry picked from commit 2b1e8d08ed869124e4a363a0c5f58999f487a4be)
Signed-off-by: Jim Hahn <jrh3@att.com>