Add setting as servers `template_source` directly

This adds an extra configuration method `template_source` for
directly setting the template source. This also renames the
other DSL method (`set_template_source`) for setting a template
source to `build_template_source`. This is a more descriptive
name now that you can set the template source via
`template_source`. This provides extra flexibility for building a
common template source and sharing it between a server and tests.

@kellyredding - Ready for review.
@jcredding why not set this default value in the ns-options DSL above?