Currently the check will use /usr/bin/tcpdump only for Ubuntu 22.04
(jammy) but it will try to use the old /usr/sbin/tcpdump path for newer
versions of Ubuntu such as 24.04 (noble) which causes the br-ex-tcpdump
service to fail to start in jobs that enable it (like multinode):
systemd[1]: devstack@br-ex-tcpdump.service: Main process exited,
code=exited, status=203/EXEC
This uses 'which' to find the path and if it is not found, use the
current check in case there is anything relying on it.
Change-Id: I034e8ca87909521c7e4f3e49996144f9c80d416d
6be68a4158