Add php-unit-test images and Vagrant template

This MR does the following:

• Adds php-unit-test images for the PHP versions 5.6, 7.0, 7.1, 7.2
• Adds a vagrant example file that can be used to run the Makefile. This is used for development of the Dockerfiles or the Makefile itself
• Adds the $(BUILD_ARGS) arguments to php-lint and magento2 images. I believe this was just an oversight.
• Splits the static-code-analysis images into separate tasks in preparation for per-language changes needed to run sonar-scanner.
• Adds sub-groups to the Makefile.
  • As we get more related images, the all task becomes large, and it is difficult to test a subset of images without calling all of them separately (e.g. make php-lint_5.6 && make php-lint_7.0 && make php-lint_7.1 && make php-lint_7.2). With this change you can now run make php-lint to build / test that entire subset of images.

php-unit-test/5.6/Dockerfile

+FROM php:5.6-alpine
+
+ARG BUILD_DATE
+ARG VCS_REF
+ARG VERSION
+LABEL org.label-schema.build-date=$BUILD_DATE \
+      org.label-schema.name="PHP 5.6 Unit Tester" \
+      org.label-schema.description="" \
+      org.label-schema.vcs-ref=$VCS_REF \
+      org.label-schema.vendor="University of Nebraska - Lincoln" \
+      org.label-schema.version=$VERSION \
+      org.label-schema.schema-version="1.0" \
+      maintainer="Alan Nelson <alan.nelson@nebraska.edu>"
+
+RUN apk add --no-cache bash nodejs nodejs-npm
+RUN curl -o /usr/local/bin/composer https://getcomposer.org/composer.phar \
+  && chmod 755 /usr/local/bin/composer
+
+CMD ["bash"]