Allow a user to specify a custom template for their container init configuration

- This allows a user using the docker_container resource to specify an
  alternate template for the init configuration. This allows for
  custom chaining of service dependencies for upstart, as well as
  environment specific benefits
- Also allow a user using docker_container resource and systemd to
  specify a custom template for the systemd socket unit file


My questions:
 - Should there be an exposed attribute? If so, should it be different for each init type?
 - Along the same lines, should init_template be specific to inittype? (i.e. upstart_template/systemd_template/sysv_template, with matching attributes?)
 - Should we also provide an attribute to allow a custom init configuration for the docker daemon itself? I can imagine needing this myself eventually, but wanted to leave it out of the scope of the PR?