vm-host parameters help is wrong

[Marian Gasparovic]

$ maas root vm-host parameters
usage: maas root vm-host parameters [--help] [-d] [-k] id [data ...]

Obtain VM host parameters
...
Note: This method is reserved for admin users.

the following arguments are required: id, data

However data is not required, it is not even supposed to do anything

`vm-host parameters id` returns all parameters in one output

`vm-host parameters id certificate` says
'certificate' is not a name=value or name@=filename pair

Eithr parameter command needs to support data and return just that field from the output or help should be corrected to not suggest `data` is a valid argument or that it is required argument.

To be clear parameters -h shows `data` as argument and parameters without specifying id says id and data are required arguments

[Wyatt Rees (wyattrees)]

We can fix this by changing how we add data to the argument parser. Currently, `data` is specified as a positional argument with `nargs="*"` and a custom type function. This forces `data` to be recognized as a required argument and specifies it as a list of key=value pairs: https://git.launchpad.net/maas/tree/src/maascli/api.py#n105

Unfortunately, argparse does not allow you to specify positional arguments as optional, unless you use `nargs="?"`. If we do use `nargs="?"`, the argument is no longer recognized as a list. However, we can then also modify the custom type converter (https://git.launchpad.net/maas/tree/src/maascli/api.py#n198) to convert a list of key=value pairs (rather than just one key=value pair, as is done now). This does remove "data" from the error message specifying required arguments, but has a side effect: whenever the `data` argument is presented for any command using it, it must be wrapped in quotes.

As a side-note: changing the API to actually use the `data` argument doesn't make a ton of sense, since the `data` argument must be presented as key=value pairs, but this specific action only requires a key.