Don't reraise Exceptions from agent driver

Since we only want drivers to raise IronicExceptions, this raises a
VendorPassthruException instead of any non-IronicException in the agent's
AgentVendorInterface.vendor_passthru() method.

Change-Id: I6a0b55e3b825ce08c1d217da287ee127989741c6
Partial-Bug: #1267693
Merge "Don't reraise Exceptions from agent driver"