Require topics and target in notify driver constructors

Pointed out by Doug.

Change-Id: Ib754e5eab58ef88c7057f2ca811088e7ef842611
Update use of stevedore

Use the NamedExtensionManager instead of the DriverManager,
since we have multiple notification drivers.

Change-Id: I33c28ae32946fbcbea955488380ae4d72dfeae55
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>