Fix broken StorPool driver

Fix broken StorPool driver

The newly added StorPool driver has a number of issues;
It imports it's own 3'rd party libs which are not in the
requirements file, it declares conf options but never registers
them, and as a result of these two things it break the ability
to generate a configuration file.

This patch adds a try_import around the import storpool calls
like we do in other drivers, and it registers the config options
properly.

We also move the api setting out of init and into the check_setup
so the service doesn't crash if somebody tries to load the driver
without the required storpool modules.

Closes-Bug: 1405023
Closes-Bug: 1405022
Closes-Bug: 1405016
Closes-Bug: 1403532

Change-Id: I61340ab7c5abcdc21dbb12cf0693da036e69e90c