Mark provider as non-defaultable

Since creating a DigitalOcean droplet requires a token to be specified
in the Vagrantfile, this provider will not work out-of-the-box with any
arbitrary Vagrantfile. This is a problem for users who have DigitalOcean
first in their provider list (e.g. by having none of the default
providers' backends installed) and a working provider later in the list.
DigitalOcean will return an "unauthorized" response and prevent Vagrant
from trying later providers, forcing the user to manually specify their
desired provider.

Additionally, as droplets begin to incur a fee as soon as they're
created, defaulting to the DigitalOcean provider for Vagrantfiles that
make no mention of it might cause users to receive an unintentional
bill--for example, in the case where they've specified their
DigitalOcean token in a user- or system-wide Vagrantfile.

This commit has no effect on the current behavior in cases where
 - a Vagrantfile contains a "config.vm.provider :digital_ocean" section
 - a user passes a provider explicitly on the command line
 - the VAGRANT_DEFAULT_PROVIDER environment variable is set

Signed-off-by: Thomas Hebb <[email protected]>

Author: tchebb

lib/vagrant-digitalocean/plugin.rb

@@ -12,7 +12,7 @@ class Plugin < Vagrant.plugin('2')
         Config
       end
 
-      provider(:digital_ocean, parallel: true) do
+      provider(:digital_ocean, parallel: true, defaultable: false) do
         require_relative 'provider'
         Provider
       end