Add dogpile.cache>=0.5.0 to requirements

dogpile.cache is the caching library of choice for caching that is
being implemented in keystone.

Change-Id: I1a8a39bb286e15853959b21edf242b28a9a8f4bb
related-blueprint: caching-layer-for-driver-calls
Merge "Add dogpile.cache>=0.5.0 to requirements"