[consensus] --evict_failed_followers is a runtime flag

Current implementation of Raft consensus treats --evict_failed_followers
as a runtime flag.  This patch adds corresponding tag for the flag.

Change-Id: Ia7ff0c4097df72c3a2d43ee166e29af5f67b5687
Reviewed-on: http://gerrit.cloudera.org:8080/17522
Tested-by: Kudu Jenkins
Reviewed-by: Bankim Bhavsar <bankim@cloudera.com>
1 file changed