Add socket keepalive options to oslo.cache

This patch specifies a set of options required to setup the
socket keepalive of the dogpile.cache's pymemcache
backend [1][2]. This setup from those options can later on
be passed to this backend.

This patch also sets up the socket keepalive object based on
the configuration options passed via oslo.config and adds it
as an argument to be passed to the selected oslo.cache backend.

Dogpile.cache will be used as an interface between oslo.cache and
pymemcache [3].

[1] https://github.com/sqlalchemy/dogpile.cache/pull/205
[2]
https://github.com/pinterest/pymemcache/commit/b289c87bb89b3ab477bd5d92c8951ab42c923923
[3]
https://dogpilecache.sqlalchemy.org/en/latest/api.html?highlight=keepalive#dogpile.cache.backends.memcached.PyMemcacheBackend.params.socket_keepalive

Change-Id: I501100e1a48cdd4e094c08046e2150405dcf371e