Add support for the Digital Ocean provider.

This commit modifies how the Chef installation hook is registered.
Rather than specify each action hook (up, provision), it hooks into
all potential actions that run the Vagrant provision middleware. This
enables custom commands that run provisioning to ensure that Chef is
installed. The Digital Ocean provider has a custom command, rebuild,
that requires this modification.

Additionally, an acceptance test for Digital Ocean has been provided and
sucessfully run. Acceptance tests for the other providers with this
modification have not been tested since the author does not have
accounts for those cloud services.