Add constants for driver lookup values

[jdolitsky]

In the Lookup method in the package github.com/cnabio/cnab-go/driver/lookup is using hardcoded strings such as "docker" to determine the driver. Those strings should ideally be made constants somewhere in this repo for other libraries to use.

[jdolitsky]

Source:

cnab-go/driver/lookup/lookup.go

Line 13 in fc67661

func Lookup(name string) (driver.Driver, error) {