Updated openstack/openstack
Project: openstack/requirements  6437e41b2b1549705eef5c7893095a181439c853
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Set boto minimum version

For supporting the S3 multipart upload in glance,
set boto minimum version to 2.12.0.

Implements: blueprint s3-multi-part-upload
Change-Id: Id57a3887256a665455fd83d3e8e752a28c5e7659