install memcached for swift via files

when swift was enabled we were installing memcached via
stack.sh, after marking it optional in files. Just use
files instead.

Change-Id: Ib8ee2d1f47254e805f4747b8aff6e89baa66913c
diff --git a/files/apts/swift b/files/apts/swift
index f298377..c52c68b 100644
--- a/files/apts/swift
+++ b/files/apts/swift
@@ -1,6 +1,6 @@
 curl
 gcc
-memcached # NOPRIME
+memcached
 python-configobj
 python-coverage
 python-dev