Don't hardcode stunnel filepath
Don't hardcode stunnel filepath

Debian systems place stunnel in /usr/bin, but we had been assuming that it was
in /usr/sbin. Now we check `which stunnel` before calling it.

Signed-off-by: Mike McClurg <d5d39725aee3103039228164deec3c7acc6f60e3@citrix.com>

simonjbeaumont/xcp-rrdd,djs55/xcp-rrdd,koushikcgit/xcp-networkd,robhoes/squeezed,djs55/xcp-networkd,koushikcgit/xcp-rrdd,djs55/xcp-rrdd,sharady/xcp-networkd,djs55/squeezed,koushikcgit/xcp-networkd,koushikcgit/xcp-rrdd,johnelse/xcp-rrdd,koushikcgit/xcp-rrdd,simonjbeaumont/xcp-rrdd,sharady/xcp-networkd,johnelse/xcp-rrdd,djs55/xcp-networkd