Dockerfiles: add Python3 and kconfiglib build dependency

This adds Python3 and the 'kconfiglib' tool to all Dockerfiles.
This is required in order to build (and/or configure) the ACRN
hypervisor.

It also adds 'findutils' to all Fedora Dockerfiles as both 'find'
and 'xargs' are used in some Makefiles and provided by this package.

Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

.travis-dockerfiles/Dockerfile.fedora26

@@ -6,6 +6,7 @@ RUN dnf -y install gcc \
                    git \
                    make \
                    vim \
+                   findutils \
                    libuuid-devel \
                    openssl-devel \
                    libpciaccess-devel \
@@ -13,7 +14,11 @@ RUN dnf -y install gcc \
                    systemd-devel \
                    libxml2-devel \
                    libevent-devel \
-                   libusbx-devel
+                   libusbx-devel \
+                   python3 \
+                   python3-pip
+
+RUN pip3 install kconfiglib
 
 WORKDIR /root/acrn