expand consul_template_config to allow contents instead of source

[shayangz]

This will allow avoiding template files when they are very trivial.

From CT config:

# This option allows embedding the contents of a template in the configuration
# file rather then supplying the `source` path to the template file. This is
# useful for short templates. This option is mutually exclusive with the
# `source` option.

Right now if source is missing the resource throws an error.