CLI: cloud-init modules -h documents unsupported --mode init

Bug Description

sudo python3 -m cloudinit.cmd.main modules -h
[sudo] password for csmith:
usage: /home/csmith/src/cloud-init/cloudinit/cmd/main.py modules
[-h] [--mode {init,config,final}]

optional arguments:
-h, --help show this help message and exit
--mode {init,config,final}, -m {init,config,final}
module configuration name to use (default: config)
csmith@uptown:~/src/cloud-init (master)$ sudo python3 -m cloudinit.cmd.main modules --mode init
Traceback (most recent call last):
File "/usr/lib/python3.5/runpy.py", line 184, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.5/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/home/csmith/src/cloud-init/cloudinit/cmd/main.py", line 835, in <module>
main(sys.argv)
File "/home/csmith/src/cloud-init/cloudinit/cmd/main.py", line 829, in main
get_uptime=True, func=functor, args=(name, args))
File "/home/csmith/src/cloud-init/cloudinit/util.py", line 2238, in log_time
ret = func(*args, **kwargs)
File "/home/csmith/src/cloud-init/cloudinit/cmd/main.py", line 631, in status_wrapper
v1[mode]['start'] = time.time()
KeyError: 'modules-init'

We need to limit options to those listed in
cloudinit.cmd.main line 606:
modes = ('init', 'init-local', 'modules-config', 'modules-final')

This bug is believed to be fixed in cloud-init in 18.1. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3078