Improve repeatability of launch script tests

Previously, tags were used for CentOS and Ubuntu images that may
change over time. This commit updates the Dockerfiles to use fixed
base images. For Ubuntu, a fixed tag is available. For CentOS we
have to resort to using the digest.

Closes gh-5397

spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/conf/CentOS/5.11-7f8a8084/Dockerfile

@@ -1,4 +1,5 @@
-FROM centos:6.7
+# CentOS 5.11 from 22/03/2016
+FROM centos@sha256:7f8a808416f712da6931ac65e4308fede7fe86bcf15364b6f63af88840fe6131
 RUN yum install -y wget && \
     yum install -y system-config-services && \
     yum install -y curl && \

spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/conf/CentOS/6.7-4f6d8f79/Dockerfile

@@ -1,4 +1,5 @@
-FROM centos:5.11
+# CentOS 6.7 from 22/03/2016
+FROM centos@sha256:4f6d8f794af3574eca603b965fc0b63fdf852be29c17d3ab4cad7ec2272b82bd
 RUN yum install -y wget && \
     yum install -y system-config-services && \
     yum install -y curl && \

spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/resources/conf/Ubuntu/trusty-20160317/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:14.04.3
+FROM ubuntu:trusty-20160317
 RUN apt-get install -y software-properties-common && \
     add-apt-repository ppa:webupd8team/java -y && \
     apt-get update && \