support string and symbol for os resource

[chris-rock]

fixes #227

[srenatus]

Not sure if I'm missing something, but my test (cd test/integation; kitchen test default-ubuntu-1404) fails:

[...]
       Finished setting up <default-ubuntu-1404> (0m0.00s).
-----> Verifying <default-ubuntu-1404>...
"You are currently running on OS family: ubuntu, OS release: 14.04"
.................................................................................[DEPRECATION] `linux_kernel_parameter(parameter)` is deprecated.  Please use `kernel_parameter(parameter)` instead.
..F..............[DEPRECATION] `belong_to_group` is deprecated.  Please use `its(:groups) { should include('root') }` instead.
........

Failures:

  1)  should eq "ubuntu"
     Failure/Error: DEFAULT_FAILURE_NOTIFIER = lambda { |failure, _opts| raise failure }

       expected: "ubuntu"
            got: nil

       (compared using ==)
     # ./test/integration/default/os_spec.rb:12:in `block (2 levels) in load'

Finished in 0.42298 seconds (files took 3 minutes 14.9 seconds to load)
106 examples, 1 failure

Failed examples:

rspec  #  should eq "ubuntu"

[srenatus]

Works in inspec shell though. Confusing. I guess my kitchen test run is not picking up the changes in inspec...?