get-pip.py dropped python 2.7 support, so we cannot use the default
get-pip.py for python 2.7 now. get-pip.py for python 2.7 is available
at https://bootstrap.pypa.io/2.7/get-pip.py.
This commit tries to use python 2.7 version of get-pip.py
for python 2.7 environment.

Depends-On: https://review.opendev.org/772486
Change-Id: I88ba370de8887a05c473fe7c19a61a319e8812d2
(cherry picked from commit 1382e933c5b2c16b276e8813fbc7cd9a1d6d4499)