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 wasn't sure either. I've added the blank line though ;o)

This seemed like the simplest way to capture the error and make it more friendly. Otherwise it requires changes in all the gems that use it (over and above bumping the version, which we'd need to do anyway).