usertools: add file-prefix option for telemetry

usertools: add file-prefix option for telemetry

Currently the dpdk-telemetry.py script connects to all running DPDK apps
consecutively. With the addition of this file-prefix argument, we can limit
the amount of information returned providing improved consumability and
precision to the user.

Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>