detect that sshfs isn't installed on target host

[SvenDowideit]

atm, it waits for a timeout

sven@x1carbon:/tmp/space path$ ./sshocker -p 8080:80 -v .:/tmp/sshfs xeon
bash: sshfs: command not found
WARN[0031] failed to confirm whether /tmp/sshfs [remote] is successfully mounted  error="failed to execute script \"wait-for-remote-ready\": stdout=\"\", stderr=\"sshfs does not seem to be mounted on /tmp/sshfs\\n\": exit status 1"
Last login: Thu Oct  8 10:00:37 2020 from 10.10.10.146