HC2VPP-176: add support for api-segment prefix

HC2VPP-176: add support for api-segment prefix

VPP can be run with custom SHM segment prefix:

sudo vpp unix {cli-listen /run/vpp/cli-vpp1.sock} api-segment { prefix vpp1 }
sudo vppctl -s /run/vpp/cli-vpp1.sock

this patch adds optional api-segment-prefix parameter to
/opt/honeycomb/config/jvpp.json file.

Example configuration:

{
  "jvpp-connection-name": "honeycomb",
  "api-segment-prefix": "vpp1",
  "jvpp-request-timeout": 5
}

Change-Id: I7ca298644891602afe7898db88bb90d59b49ce4d
Signed-off-by: Marek Gradzki <mgradzki@cisco.com>