Fix for memcache middleware configuration

The documentation rightly said to use "memcache_max_connections", but
the code was looking for "max_connections", and only looking for it in
proxy-server.conf, not in memcache.conf as a fall back.

This commit brings the code coverage for the memcache middleware to
100%.

Closes-Bug: 1252893
Change-Id: I6ea64baa2f961a09d60b977b40d5baf842449ece
Signed-off-by: Peter Portante <peter.portante@redhat.com>