Make gnocchi-base build from source

gnocchi-base requires the gcc c++ compiler to build panda, whatever
that is.

Change-Id: Icb4519cf4881390f6958324eb14fb0009fca336e
Paritally-Implements: blueprint gate-source-builds

docker/centos/source/base/Dockerfile

@@ -5,6 +5,7 @@ MAINTAINER Kolla Project (https://launchpad.net/kolla)
 RUN yum install -y \
     epel-release \
     gcc \
+    gcc-c++ \
     git \
     libffi-devel \
     libxml2-devel \

docker_templates/base/Dockerfile.j2

@@ -145,6 +145,7 @@ RUN yum update -y \
     && yum install -y \
         epel-release \
         gcc \
+        gcc-c++ \
         git \
         libffi-devel \
         libxml2-devel \
@@ -185,6 +186,7 @@ RUN apt-get update \
 
 RUN apt-get install -y --no-install-recommends \
         gcc \
+        gcc-c++ \
         git \
         ldap-utils \
         libffi-dev \