Provide a way to share hosts certificates in containers

[mariash]

Concourse users would like to have a way to share their hosts common CA certificates inside of containers. Currently even though they install them on VM or configure bosh to install certs for them they have to include common CA certificates in every image their container is running.

We think that sharing certificates from host might be a feature provided by Garden. Same way garden manages /etc/hosts inside of containers it can also manage /etc/ssl/certs. And since it is a common problem for both Concourse and Diego it would be nice if that feature was baked in the backend.

Related Concourse issue: concourse/concourse#1027

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/146397903

The labels on this github issue will be updated when the story is started.

[mariash]

same as #83