Allow the implemention cookbook's template for setenv.sh

[sparuvu]

The setenv.sh.erb currently supports the adding java_settings and env_settings. While the template does support all the valid property formats (for example -Xmx256, -XX:MaxPermSize=, -DAbc etc) it requires the consumers to configure their properties in certain way for example to set the MaxPermSize the property name has to be MaxPermSize= which the consumer can either set it in their configuration or a recipe/library in the implementation cookbook can do that. But the better approach would be to allow the consumers to point to their own template and fallback to the current template if none is mentioned explicitly while calling the setenv LWRP. Another advantage with allowing consumers to point to their own template is that they can add some custom documentation to their setenv.sh file about why they configured certain property.

[EdwardSkoviak]

After #34 is merged, we can use the poise template feature to support this in an improved way(refer to linked doc).