Pin get-pip.py to 3.2

As get-pip.py evolves based on pip 10, while we're still using
pip 9, changes in the way it can be used are causing problems.

For example, the ability to use --download is no longer there.

As such, let's pin to a known good version and leave it at that
until we no longer need to use this script. Version 3.2 maps to
pip 7.1.2 which fulfills our needs.

Change-Id: I6aa297ee6e1ca3a76f016a6a0c3d4a735433a9b2
Closes-Bug: 1779534

run_tests_common.sh

@@ -90,7 +90,7 @@ eval sudo ${pkg_mgr_cmd} ${pkg_list}
 # Install pip
 if ! which pip &>/dev/null; then
     curl --silent --show-error --retry 5 \
-        https://bootstrap.pypa.io/get-pip.py | sudo python2.7
+        https://bootstrap.pypa.io/3.2/get-pip.py | sudo python2.7
 fi
 
 # Install bindep and tox