Drop py35 tests
Drop py35 tests

Target python runtime for Stein is py36 [1]. So, I will drop py35
tests.

[1] https://governance.openstack.org/tc/reference/runtimes/stein.html#python-runtime-for-stein

Change-Id: I3b6db2463864635246ccb4a551898861b890a818
Signed-off-by: Trinh Nguyen <f87cc599f9e22238e9ddceba1aa34d0d1ff5071f@gmail.com>

openstack/python-searchlightclient,openstack/python-searchlightclient