Bump stevedore to >=0.14

Current savanna code depends on entry_point_target that was added by
a8150369a045f08f86db00e49a45bda4594fd2d5, which is available in >= 0.13

It depends on https://review.openstack.org/#/c/69496/

Change-Id: I8c3c4ebac116be4bc20429f9f9c82feac67c8802
Closes-Bug: #1273459
Merge "Bump stevedore to >=0.14"