Let the import drain loop accumulate a line

The loop that drains the importer's remaining output reads a byte at a
time but clears currline on every iteration, one level out from where
the earlier loop clears it.  currline is therefore never longer than a
single byte, so the percentage and ERROR: branches can never match and
the tail of an import is silently discarded.

Clear it only once a line has been consumed, as the earlier loop does.

confluent_server/confluent/osimage.py

@@ -1301,7 +1301,7 @@ class MediaImporter(object):
                         self.error = self.error.decode('utf8')
                     self.phase = 'error'
                     return
-            currline = b''
+                currline = b''
             a = await wkr.stdout.read(1)
         if self.oscategory:
             defprofile = '/opt/confluent/lib/osdeploy/{0}'.format(