Allow user to specify a configuration file in VMPOOLER_CONFIG_FILE

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.

spec/unit/providers/vsphere_spec.rb

@@ -820,7 +820,7 @@ EOT
 
       it 'should honor the insecure setting' do
         pending('Resolution of issue https://github.com/puppetlabs/vmpooler/issues/207')
-        config[:vsphere][:insecure] = false
+        config[:providers][:vsphere][:insecure] = false
 
         expect(RbVmomi::VIM).to receive(:connect).with({
           :host     => credentials['server'],