Allow user to specify a configuration file in VMPOOLER_CONFIG_FILE variable

[amcdson]

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.

[logicminds]

@adamdav There is a failure in the test because the vmpooler.yaml cannot be found. You might want to mock that file in that test.

 1) Vmpooler .config when no config is given defaults to vmpooler.yaml
     Failure/Error: default_config = YAML.load_file(default_config_file)
     
     Errno::ENOENT:
       No such file or directory @ rb_sysopen - /home/travis/build/puppetlabs/vmpooler/spec/fixtures/vmpooler.yaml
     # ./spec/unit/vmpooler_spec.rb:17:in `block (4 levels) in <top (required)>'

[glennsarti]

This looks great, AND TESTS!!!! awesome. If you rebase onto master, that'll fix the travis errors.

[amcdson]

@glennsarti branch is rebased