MB-25591 Use only Python 2.6 features in kv_trace_dump
MB-25591 Use only Python 2.6 features in kv_trace_dump

CentOS 6 only supports upto Python 2.6, not Python 2.7 and therefore
does not have `subprocess.check_output`.

Change-Id: Iaa8809786eb21bdb630094601e480cb7b0d35e39
Reviewed-on: http://review.couchbase.org/82017
Tested-by: Will Gardner <5d7e5a4e8e63c739e07f011adf51f2e51dd3ec09@rdner.io>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Oliver Downard <2b519a6943d9144e0afb0b62d71fdc5e98fa8017@couchbase.com>
Reviewed-by: James Harrison <5c25a7b3db09830813688775bfc8e6de736a230c@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine