load values from ~/.config/equinix/metal.yaml

[displague]

Relates to #63

Loads ~/.config/equinix/metal.yaml (as configured by the metal-cli) as an initial config value source. METAL_CONFIG can be set in the environment to supply an alternate config file.

This makes the following fields configurable via file:

	Token     string `json:"token,omitempty"`
	AuthToken string `json:"auth-token,omitempty"`
	Facility  string `json:"facility,omitempty"`
	Metro     string `json:"metro,omitempty"`
	OS        string `json:"operating-system,omitempty"`
	Plan      string `json:"plan,omitempty"`
	ProjectID string `json:"project-id,omitempty"`

token is preferred to auth-token. Either field will set --metal-api-key.

The field names in the file match what the metal-cli expects (in 0.6.0-alpha3, these names could change before final release).

With the metal-cli configuration and the existing docker-machine-driver defaults, no arguments need to be provided to create a node.