[GH-1390] Remove redundant calls to Fog.credentials

If you use the supported means to create services such as:

    Fog::Service.new(:provider => "Example")

It already loads the contents of `~/.fog` and filters the values based
on if they are recognised or required.

That means in a number of services that are initialising based on
`options` or a value from `Fog.credentials` both are the same.

The `options` should have been loaded and merged from credentials in the
service "builder".
Fix syntax errors in an OpenStack example script
[GH-2793] Exclude examples from documented methods

Example code in various providers directories is getting picked up by
Yard when documenting such that a number of global constants and helpers
are appearing to be part of fog.

This excludes them using `.yardopts`

It also excludes generated documentation from provider subdirectories
from git.
Merge pull request #2795 from L2G/openstack-example-syntax-err

Fix syntax errors in an OpenStack example script
Merge pull request #2792 from fog/1390_remove_credential_calls

[GH-1390] Remove redundant calls to Fog.credentials