cloudapi: User customization

[croissanne]

Add support for the user stage to the composer-api as an optional customization.

type UsersStageOptionsUser struct {
	UID         *int     `json:"uid,omitempty"`
	GID         *int     `json:"gid,omitempty"`
	Groups      []string `json:"groups,omitempty"`
	Description *string  `json:"description,omitempty"`
	Home        *string  `json:"home,omitempty"`
	Shell       *string  `json:"shell,omitempty"`
	Password    *string  `json:"password,omitempty"`
	Key         *string  `json:"key,omitempty"`
}

Fields that should be supported per user:

• Key
• Groups

Maybe:

• ?Home?
• ?Password? (not sure about this one, erring on not)

And could we switch the API tests, so that they generate an ssh key, embed it, and then ssh into it using that. Rather than generating the keys via the platform? (Hope that's possible for all 3 cloud platforms).