Expose API for fetching a single driver

The API currently allows drivers to be listed, but not fetched
individually.  This patch allows drivers to be retrieved by name.
While the current implementation offers only a modest performance
advantage over simply listing all drivers, this is required by Pecan
to enable us to expose further routes beneath
/v1/drivers/<driver_name>/.

Closes-Bug: #1294869

Change-Id: Id1a0868c58abb6ab10c2e5ca65559c91795dba26
Drivers may expose a top-level passthru API

This allows drivers to implement a vendor method which will be executed on a
random conductor upon any POST to
/v1/drivers/<driver_name>/vendor_passthru/<method_name>, where <method_name> is
an arbitrary path segment which will be passed to the driver_vendor_passthru
method on the vendor interface.

Change-Id: Ia0866e9ca59c8eba7e52eabeb8c88076c54ebbfc