CBQE-1187 replace cygwin command with cmd /c for backup automation

Change-Id: If04211d992822e820194aea38db97f517d345f34
Reviewed-on: http://review.couchbase.org/27079
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>