[storage] Add an SQL storage driver

This commits adds the possibility to back Tiller (or the future
Tiller-less Helm CLI) with any SQL database (only postgres has been
tested so far) to store release information.

The main motivation for this commit was to use a storage backend that
would allow releases larger that 1MB in size (ConfigMap or Secret
drivers don't, because of limits on value size in the underlying etcd
key-value store).

Signed-off-by: Étienne Lafarge <etienne.lafarge@gmail.com>

Co-authored-by: Elliot Maincourt &lt;e.maincourt@gmail.com&gt; <(@emaincourt)>
Co-authored-by: Paul Borensztein &lt;hi@0x01.fr&gt; <(@commit-master)>