Work with pymongo 3.0
Work with pymongo 3.0

pymongo 3.0 renamed mongos_enum to read_pref_mode_from_name which
was causing the unit tests to fail.

Change-Id: Iaa7fd7221c2e6c865633ef342e6b83304a1de655
Closes-Bug: 1441393
(cherry picked from commit 7c3fe6acaef6fc283c383fb79f06e388df6e6926)

ilay09/keystone,cernops/keystone,openstack/keystone,vivekdhayaal/keystone,mahak/keystone,jonnary/keystone,rajalokan/keystone,vivekdhayaal/keystone,jamielennox/keystone,ajayaa/keystone,cernops/keystone,dstanek/keystone,klmitch/keystone,mahak/keystone,jamielennox/keystone,idjaw/keystone,roopali8/keystone,jonnary/keystone,rajalokan/keystone,dstanek/keystone,ilay09/keystone,dims/keystone,vivekdhayaal/keystone,dstanek/keystone,idjaw/keystone,rajalokan/keystone,ilay09/keystone,takeshineshiro/keystone,maestro-hybrid-cloud/keystone,himanshu-setia/keystone,takeshineshiro/keystone,dims/keystone,mahak/keystone,himanshu-setia/keystone,openstack/keystone,maestro-hybrid-cloud/keystone,ajayaa/keystone,roopali8/keystone,openstack/keystone,klmitch/keystone