Multiple DNS-related fixes and improvements for docker::run and docker::service

Included changes:
* Fixed handling of `$docker::dns`: One can specify that parameter, but the value was not actually included in the different configuration files. The parameter was added to `docker::service`, and the three templates were modified to include it. Unit tests were added to prevent regressions.

* Added a `$docker::run::dns_search` parameter: Modified `docker::run` and the corresponding templates. Also includes unit tests.

* Cleaned up and unified the formatting of the configuration templates.

* Added a `$docker::dns_search` parameter: Like above, but for the docker daemon instead of individual containers. Includes unit tests.