This commit adds a new flag to the `vagrant validate` command which
allows users to completely ignore the provider block of a config file.
This is useful for when you are running `vagrant validate` in CI and
don't want to install a valid provider to check the syntax of your
Vagratnfile. When the flag is invoked, a warning will be displayed
saying that the provider block will be ignored and not validated.
Prior to this commit, the `vagrant validate` command would only validate
the first machine in a vagrant file. This commit improves that by
validating all known machines in the environment. If one is not found,
it will properly throw an exception instead of a stacktrace.