There were several implementations of hashing the content
of a file in cli/recon.py and common/middleware/recon.py.
This patch relocates one implementation (_hash_for_ringfile,
introduced in the Related Change) to common/utils.py and
refactors recon cli and middleware to use that function.
Also improves use of mocking in the unit tests to eliminate passing
custom file opener functions to the ReconMiddleware get_ring_md5
and get_swift_conf_md5 methods.
Related-Change: I9623752c3cd2361f57864f3e938e1baf5e9292d7
Change-Id: Iaad88e49aadeb28f614aafa1e9596fe07ce9793a