Fix netcat option in udp_check.sh for CentOS/RHEL

-w (timeout) option doesn't do anything in nmap-ncat (default netcat in
CentOS/RHEL) for UDP datagrams, and nmap-ncat has a default idle timeout
set to 2 seconds.
We can get the same behavior as netcap-openbsd (Debian/Ubuntu) by
setting that idle timeout (-i) option to 1 second.

This commit detects the flavor of the netcat binary (nmap vs other) and
uses it to adapt the parameters.

Story: 2007688
Task: 39800

Change-Id: I0100aaa428477f011bd39a90dd4ec98199b4bebc

octavia/amphorae/backends/utils/udp_check.sh

@@ -1,4 +1,16 @@
 #!/bin/bash
+
 nc_cmd=`which nc`
-$nc_cmd -uzv -w1 $1 $2 > /dev/null
+
+nc_flavor=$($nc_cmd --version 2>&1 | grep -o nmap)
+case "$nc_flavor" in
+nmap)
+    nc_flavor_opts="-i1"
+    ;;
+*) # default, probably openbsd
+    nc_flavor_opts="-w1"
+    ;;
+esac
+
+$nc_cmd -uzv $nc_flavor_opts $1 $2 > /dev/null
 exit $?