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>
Merge pull request #8 from dhellmann/messaging-stevedore-updates

Update use of stevedore