Don't index logs with DEBUG log level.

ElasticSearch has a hard time performing queries on large amounts of
data. It must load the fields it is searching on into memory and that
can cause the node to use all of the memory allocated to it which in
turn causes a lot of garbage collection essentially taking the node
offline. Filter out DEBUG log messages in the devstack logs to reduce
the amount of data that must be loaded into memory when performing
searches.

Change-Id: Icfe9c8c17ccef4f9379c774eef791f43463dcf6d
Merge "Don't index logs with DEBUG log level."