httpserver: fix command line arguments

Currently no command line arguments are declared as accepting values,
for example --port 30000 will result in "port" being set but to an
empty value and 30000 will be ignored.

This does not affect parameters passed via cloud-init.

Reported-by: Or Cohen <orc@fewbytes.com>
Analysed-by: Calle Wilund <calle@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>