Remove customized require behaviors and modify the bin executable
to check for missing tools that Vagrant expects to exist when
running outside of an installer.
Prior to this commit, if a user ran the `vagrant ssh -c CMD` command, it
would not allow the user to configure pseudo-terminal allocation. This
commit introduces a -t flag for the `vagrant ssh` command which defaults
to true if not specified.
This works by now calling the `:ssh` action on the provider. This action
is allowed to do whatever it pleases, but should at some point probably
call the `SSHExec` built-in middleware.
The `SSHExec` built-in middleware was added. This uses the information
returned by `Machine#ssh_info` and uses the `Vagrant::Util::SSH` helper
to exec into the remote machine. The provider should do any work upfront
in verifying that the machine is ready to be SSHed into.
