Disable plugin if a URL isn't specified
Disable plugin if a URL isn't specified

Rather than throw an error, this change disables vagrant-cookbook-fetcher.
Without this, any Vagrantfile that doesn't use vagrant-cookbook-fetcher will
throw an error, preventing you from using vagrant without configuring this
plugin.

clintoncwolfe/vagrant-cookbook-fetcher,clintoncwolfe/vagrant-cookbook-fetcher