Install which if its not installed in base

Some variants of RHEL containers are missing the 'which' command.
Percona's software used by mariadb depends on which, but doesn't have
a hard dependency requirement since it is usually installed in the
distribution.  To make mariadb work multinode on RHEL, install which
all the time for RPM distros.

Change-Id: I2c6aaa77a1fc8b7028b658a0dfd1935c1f1d738e
Partially-Implements: blueprint rhel-based-image-support

docker/base/Dockerfile.j2

@@ -148,6 +148,7 @@ RUN yum install -y \
         python-webtest \
         python-werkzeug \
         python-wsme \
+        which \
     && yum clean all
 
     # Endif for install_type binary
@@ -174,6 +175,7 @@ RUN yum update -y \
         python-oslo-policy \
         sqlite-devel \
         tar \
+        which \
     && yum clean all
 
     # Endif for install_type source