kolla-build monasca on centos errors with rake requires Ruby version >= 2.2

[Eric Miller]

When build Kolla containers using kolla-build (Stein version, from source) on CentOS 7, the monasca-grafana container fails to build because of the following error:

INFO:kolla.common.utils.monasca-grafana:Step 12/15 : RUN gem install rake fpm && curl -sSL -o /tmp/monasca-grafana.tgz ${monasca_grafana_url} && mkdir -p ${monasca_grafana_build_path} && tar --strip 1 -xvf /tmp/monasca-grafana.tgz -C ${monasca_grafana_build_path} && rm -f /tmp/monasca-grafana.tgz && cd ${monasca_grafana_build_path} && go run build.go setup && npm install && go run build.go build package && mv ${monasca_grafana_build_path}/dist /grafana_pkgs && rm -rf /grafana
INFO:kolla.common.utils.monasca-grafana: ---> Running in 898cb01c84d4
INFO:kolla.common.utils.monasca-grafana:ERROR: Error installing rake:
INFO:kolla.common.utils.monasca-grafana: rake requires Ruby version >= 2.2.
INFO:kolla.common.utils.monasca-grafana:

Seems that CentOS 7 installs Ruby version 2.0

# ruby -v
ruby 2.0.0p648 (2015-12-16) [x86_64-linux]

I'm researching the best way to get a later version of Ruby installed on CentOS 7, but in case someone else knows the best approach, the Dockerfile.j2 file for monasca-grafana should be updated.

Eric