[google|compute] Added read metadata from file functionality

The patch adds the ability to read metadata from files allowing users to specify startup scripts or arbitrary metadata for instances in separate files.
Please be consistent with your block style when nesting blocks. This should either be:

```ruby
metadata.merge!(metadata) {|k,v| File.open(v,"r") {|f| f.read }}
```

or

```ruby
metadata.merge!(metadata) do |k,v|
  File.open(v,"r") do |f|
    f.read
  end
end
```