Suppress Fog progress bar in quiet mode

The only ways of suppressing the progress bar in the
current release of Fog are:

* Not be writing to a terminal
* Setting the :vcloud_director_show_progress option on each Fog connection created.
* Setting the 'vcloud_director_show_progress' FOG_RC credential option to false.

Not writing to a terminal (or faking this) is not sensible -- we're a CLI tool first and foremost.

Setting the option on each connection is problematic in the current code-base. We do
not use a single Fog connection object throughout, so would need to either:

* ensure that the --quiet option is passed throughout our code
* update our code to DI a single Fog connection object from the top level

The FOG_RC credentials option can be overridden by writing to Fog.credentials, as documented
in https://github.com/fog/fog-core/blob/master/lib/fog/core/credentials.rb#L6-L24

This, whilst a little non-intuitive, is useful to us as it sets the option globally:
if we create multiple Fog connection objects, they all will adopt this behaviour.
Stub ::Fog.credentials call disabling progress bar

The previous commit broke the unit tests, since we were talking to Fog
proper to set the credentials option to disable the progress bar.

Our setting of FOG_CREDENTIAL to a bogus value, which is present to
protect us from accidentally connecting to a real organisation,  picked
this up.

As discussed in the previous commit message, setting the Fog.credentials
configuration option is the only useful method we have at our disposal
to toggle display of the progress bar.

However, since this is a unit test we must stub out this call to the
external Fog library, and just ensure that we are setting the option we
know is required.