Add libtool to CentOS6 docker configuration

Signed-off-by: Alexey Neyman <stilor@att.net>

testing/docker/centos6/Dockerfile

@@ -3,7 +3,7 @@ ARG CTNG_UID
 ARG CTNG_GID
 RUN groupadd -g $CTNG_GID ctng
 RUN useradd -d /home/ctng -m -g $CTNG_GID -u $CTNG_UID -s /bin/bash ctng
-RUN yum install -y autoconf gperf bison flex texinfo help2man gcc-c++ patch \
+RUN yum install -y autoconf gperf bison flex texinfo help2man gcc-c++ libtool libtool-bin patch \
     ncurses-devel python-devel perl-Thread-Queue bzip2 git wget xz unzip
 RUN wget -O /sbin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.1/dumb-init_1.2.1_amd64
 RUN chmod a+x /sbin/dumb-init