Bug 16105: Do not initialize the memory cache if not needed

Bug 16105: Do not initialize the memory cache if not needed

Cache::Memory is loaded and Koha::Cache::_initialize_memory is called
even if a memcached cache has been correctly initialize, it does not
make sense.
It should only be initialize if needed.

Test plan:
Correctly configure a memcache server and confirm that the memory cache
is not initialize.
Do not configure correctly a memcache server and confirm that the cache
system default on Cache::Memory

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>