Use chartutil to process requirements

[amir]

This PR adds necessary requirements processing to InstallRelease.

Before this PR, following request will dispatch mariadb as a dependency to Tiller by following command:

curl -v -H "Content-type: application/json" -d '{"repo":"stable", "namespace": "default", "chart": "wordpress", "version": "0.8.7", "values": {"mariadb": {"enabled": false}, "externalDatabase": {"password": "password", "rootPassword": "password"}}}' localhost:5000/api/v1/releases

Even though mariadb is explicitly disabled using values.

This PR uses chartutil to exclude disabled dependencies and also imports values from child to parent.

[imduffy15]

This looks good to me. @darkcrux any reason to not merge?

[amir]

@imduffy15 I ended up creating https://github.com/amir/skeg which if I remember correctly kinda works as-is. During developing that I discovered a few issues such as https://github.com/helm/helm/pull/4106 which you should to be aware of (that one hasn't been merged yet either).

[imduffy15]

Nice! Thanks for sharing @amir