CBQE-1011 Customized loop setup for infinite run time

By this change, we can assign how many phases we want to loop
forever to simulate the stree test or infinite run time of the
test. The new json file will be:
{
    "name" : "simple",
    "desc" : "simple key-value test",
    "loop": "[2:]",
    "phases" : {
                "1" :
                {
                    "name" : "create_buckets",
                    "desc" :  "create buckets",
                },
                "0" :
                {
                    "name" : "reb_in_one",
                    "desc" :  "RB-1",
                    "cluster" :  {"add" : "1"}
                },
                "2" :
                {
                    "name" : "simple_load",
                    "desc" :  "load items at 1k ops",
                    "workload" : "s:100,ccq:simplekeys,ops:100",
                    "template" : "default",
                    "runtime" : 60
                },
                "3" :
                {
                    "name" : "simple_get",
                    "desc" :  "get items at 1k ops",
                    "workload" : "g:100,coq:simplekeys,ops:100",
                    "template" : "default",
                    "runtime" : 60
                }
        }
}
This means phase 2 and phase 3 will be looping infinitely because
you set phases to loop range is [2:]. This range expression borrows
the idea of python list index range idea. So format like [2], [2:3],
[:2] and [2:] are all supported
Change-Id: Ic23f5d0a0f6a78ec8d27ce825e724cc9d17eeef0
Reviewed-on: http://review.couchbase.org/27787
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>