Prior to this commit the docs and examples used 'company.com'
(a real domain). This commit changes those occurrences to
'example.com', which is a IANA-managed reserved domain.
variable
Previously, there were two ways to configure Vmpooler, either by
changing the contents of vmpooler.yaml or by assigning the raw YAML
to the VMPOOLER_CONFIG environment variable. This commit adds a new
environment variable called VMPOOLER_CONFIG_FILE that can be assigned
the name of a config file to use. Also fixes#240 by whitelisting the
Symbol class when calling YAML.safe_load in Vmpooler.config.