Check for Vagrant early

[nixjdm]

There's a check for the version of vagrant in the vagrantfile, but if vagrant isn't installed at all then there is a nasty error output that looks like this:

Traceback (most recent call last):
  File "/home/joe/miniconda3/envs/rambo/bin/rambo", line 11, in <module>
    load_entry_point('Rambo-vagrant', 'console_scripts', 'rambo')()
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/joe/wrkspc/rmb-prjcts/rambo/rambo/cli.py", line 127, in destroy_cmd
    app.destroy(ctx)
  File "/home/joe/wrkspc/rmb-prjcts/rambo/rambo/app.py", line 197, in destroy
    vagrant_general_command('destroy --force')
  File "/home/joe/wrkspc/rmb-prjcts/rambo/rambo/app.py", line 509, in vagrant_general_command
    _invoke_vagrant(cmd)
  File "/home/joe/wrkspc/rmb-prjcts/rambo/rambo/app.py", line 69, in _invoke_vagrant
    with Popen(cmd, stdin=slaves[0], stdout=slaves[0], stderr=slaves[1]) as p:
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/subprocess.py", line 709, in __init__
    restore_signals, start_new_session)
  File "/home/joe/miniconda3/envs/rambo/lib/python3.6/subprocess.py", line 1344, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'vagrant': 'vagrant'

This should be avoided with an early check and notification if vagrant isn't installed :)