Ban plaintext passwords

In version 0.7.0 we deprecated the use of plaintext passwords in FOG_RC
in a conscious move to promote that as bad practise. Interaction with
vcloud-tools should now be done by supplying a token, which can be
gained using `vcloud-login` (which will accept a password on STDIN).

This involved:

 - Moving check_plaintext_pass into Vcloud::Core::Fog
 - Calling check_plaintext_pass from check_credentials
 - replacing call to check_plaintext_pass from Vcloud::Core::Fog::Login
   with call to Vcloud::Core::Fog.check_credentials
 - Rescuing call to Vcloud::Core::Fog.check_credentials in
   `vcloud/core/fog.rb` to provide a nice error message to the user

Happy to be guided on better ways to do this - it's my first solo foray
into vcloud-tools.

Rubocop and Specs pass. Not much changed in the test apart from the
expected error message, which I have lengthened slighly to describe the
reason behind the error as well as what it means.
I think we should be careful to separate our concerns here - vCloud Core is a library, so any code relating to stderr, stdout and exiting should be handled by the gems that require it.

You're right, it will need changes in the other gems (though not all print backtraces currently).

I've just remembered there's a [story](https://www.pivotaltracker.com/story/show/76427142) that you might want to consider as part of this.