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