Add a default template path for when we can't find a pipeline_template

[justinharringa]

Add an option to specify a default template path for when we can't find a pipeline_template

[AboorvaDevarajan]

@justinharringa Justin, we are using the config-driven-plugin to version-control jenkinsfiles, but we couldn't always have a pipeline_template configured in the project repository so I patched the plugin to add an additional configuration, so I thought it will be good to open a PR.
#195

This is just an initial try to get this working. please, let me know your comments. Thanks.

[justinharringa]

Hey there @AboorvaDevarajan. I'm traveling a bit at the moment and will try and get some comments on the PR. Regardless, I definitely appreciate the contribution! 😄