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/vmpooler_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+
+describe 'Vmpooler' do
+  describe '.config' do
+    let(:config_file) { File.join(fixtures_dir, 'vmpooler2.yaml') }
+    let(:config) { YAML.load_file(config_file) }
+
+    before(:each) do
+      ENV['VMPOOLER_DEBUG'] = 'true'
+      ENV['VMPOOLER_CONFIG_FILE'] = nil
+      ENV['VMPOOLER_CONFIG'] = nil
+    end
+
+    context 'when no config is given' do
+      it 'defaults to vmpooler.yaml' do
+        default_config_file = File.join(fixtures_dir, 'vmpooler.yaml')
+        default_config = YAML.load_file(default_config_file)
+
+        Dir.chdir(fixtures_dir) do
+          expect(Vmpooler.config[:pools]).to eq(default_config[:pools])
+        end
+      end
+    end
+
+    context 'when config variable is set' do
+      it 'should use the config' do
+        ENV['VMPOOLER_CONFIG'] = config.to_yaml
+        expect(Vmpooler.config[:pools]).to eq(config[:pools])
+      end
+    end
+
+    context 'when config file is set' do
+      it 'should use the file' do
+        ENV['VMPOOLER_CONFIG_FILE'] = config_file
+        expect(Vmpooler.config[:pools]).to eq(config[:pools])
+      end
+    end
+  end
+end