Fix failing unit spec

The spec "should return a ResourceError exception if remote URI is
invalid" expects the URI "http://example.com/file.tgz" to be invalid and
to throw a ResourceError exception but on my machine that URI is valid,
returns a 404 code, and throws a ResourceNotFound error. Fixed to use a
URI that's actually invalid.
Allow running subset of unit tests

You can now pass a directory pattern to the spec:unit rake task, as in
"bundle exec rake spec:unit[bosh_*]" which will run all specs in
directories starting with "bosh_*".  Omitting the parameter will run all
specs as before.
Start adding specs for bosh_tier3_cpi