fix udp_ping api naming error

fix udp_ping api naming error

Error messages:
ERROR:VAPI CPP GEN:Cannot find reply to message `udp_ping_add_del_req'
ERROR:VAPI CPP GEN:Cannot find reply to message `udp_ping_export_req'
ERROR:VAPI C GEN:Cannot find reply to message `udp_ping_add_del_req'
ERROR:VAPI C GEN:Cannot find reply to message `udp_ping_export_req'

Api was *_req/*_resp, but when explicitely declared, the reply should
then have been *_req_reply.
Both api response are empty, so I propose to remove them, and use
autoreply

API changes:
rename udp_ping_add_del_req -> udp_ping_add_del
rename udp_ping_export_req -> udp_ping_export
delete udp_ping_add_del_reply (empty response)
delete udp_ping_export_reply (empty response)
vl_api_version 1.0.0 ->2.0.0

Change-Id: Id3456c8101a7689fbc56dc2d9076625c2b3af0ad
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>