trivial-httpd: add option to specify the port

I use the trivial httpd server locally. Each time I restart the
server, I end up modifying manually the config file for other repos so
to point to the correct port. In this way I can just re-use the same
port.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

tests/test-commit-sign.sh

@@ -56,8 +56,9 @@ cd ${test_tmpdir}
 mkdir ${test_tmpdir}/httpd
 cd httpd
 ln -s ${test_tmpdir}/ostree-srv ostree
-${CMD_PREFIX} ostree trivial-httpd --autoexit --daemonize -p ${test_tmpdir}/httpd-port
+${CMD_PREFIX} ostree trivial-httpd --autoexit --daemonize -P 18081 -p ${test_tmpdir}/httpd-port
 port=$(cat ${test_tmpdir}/httpd-port)
+assert_streq $port 18081
 echo "http://127.0.0.1:${port}" > ${test_tmpdir}/httpd-address
 cd ${oldpwd}