Attach multiple tags to logstash events.

* modules/openstack_project/files/logstash/log-pusher.py: If a list of
tags is provided under a source file configured in the yaml config
attach those tags to the log events generated from that source file.

Example yaml:
  source-files:
    - name: console.html
      retry-get: True
      tags:
        - foo
        - bar

Change-Id: Ib74abad2d06d3e52e5b21b0fb38033f9474ab4e4
Reviewed-on: https://review.openstack.org/29808
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins