Add 'python-memcached' requirement.

When set to use `memcached` as a caching system, `keystoneauthmiddleware`
directly imports a `memcached`-related submodule of `oslo_cache`.
The problem is that neither `oslo_cache`, nor the `dogpile` package it
uses (which is configurable with a number of backends) actually directly
require `python-memcached` by default, and thus it will not be installed
at all leading to an ImportError.

requirements.txt

@@ -20,6 +20,8 @@ pbr
 psutil
 pyOpenSSL
 python-keystoneclient
+# required by `oslo_cache` when imported by `keystonemiddleware`
+python-memcached>=1.56
 python-barbicanclient
 python-swiftclient>=3.2
 git+https://github.com/cloudbase/pywinrm.git@requests#egg=pywinrm