vswitchd: Refresh SSL keys and certificates more frequently.

vswitchd: Refresh SSL keys and certificates more frequently.

Until now, the ovs-vswitchd main loop has refreshed keys and certificates
from their files only when the database changes.  This works fine if new
keys and certificates are installed with new file names, because the update
to the database to point to the new files will cause them to be read.  But
if the new keys and certificates are copied over the existing files, then
the delay until they are read is indefinite.

This commit fixes the problem by changing the SSL configuration so that it
is rechecked on every trip through the ovs-vswitchd main loop.

Bug #2921.