fluentd image build failed

[raja]

when build fluentd image with base_distro=centos, the build will failed for the newest gem public_suffix(3.0.0) requires ruby>2.1.0, while the ruby version on the centos is 2.0.0.648, so we should install public_suffix==2.0.5 manual before install the fluent-plugin-kubernetes_metadata_filter

see: https://rubygems.org/gems/public_suffix

the error log:

```
INFO:kolla.image.build.fluentd:ERROR: Error installing fluent-plugin-kubernetes_metadata_filter:
INFO:kolla.image.build.fluentd: public_suffix requires Ruby version >= 2.1.
INFO:kolla.image.build.fluentd:
```

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/498339
Committed: https://git.openstack.org/cgit/openstack/kolla/commit/?id=aeb6e74bc35ee15ba6e5c43a1e16f245717ea444
Submitter: Jenkins
Branch: master

commit aeb6e74bc35ee15ba6e5c43a1e16f245717ea444
Author: jiangpch <email address hidden>
Date: Mon Aug 28 03:01:12 2017 -0400

    fix image build error of fluentd

    the newest public_suffix gem required ruby>2.1.0, but the ruby on
    centos' release is 2.0.0, we should install public_suffix==2.0.5
    manual to avoid the ruby version not matched error.

    Closes-Bug: #1713416

    Change-Id: I725294da206a73b759027d3123d140bddafcfdfb

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/kolla 5.0.0.0rc1 release candidate.