CBQE-1267: add variable caching

at some points in test we want to save some state and refer to later. to address this we will have a new cache tag within a phase with a key named reference that can be used later. ie

"0" :
{
    "name" : "cache curr items",
    "cache" : {"bucket" : "default",
    "stat" : "curr_items",
    "reference" : "phase2_default_curr_items"}
},

caches curr_items. then refer to later in postcondition

"1" :
{
    "name" : "load",
    "workload" : {"spec" : "s:100,ops:1000",
                  conditions" : "post:curr_items = $phase2_default_curr_items"},
    "runtime" : 120,
}

Change-Id: Idc97c68b3344239002075d42c909e144e3c5e177
Reviewed-on: http://review.couchbase.org/26074
Tested-by: Chisheng Hong <chisheng@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>