Release changes for 3.1.0

[jdeathe]

• Updates source image to 2.6.0.
• Updates CHANGELOG.md to simplify maintenance.
• Updates README.md to simplify contents and improve readability.
• Updates README-short.txt to apply to all image variants.
• Updates Dockerfile org.deathe.description metadata LABEL for consistency.
• Updates bootstrap and wrapper supervisord configuration to send error log output to stderr.
• Updates bootstrap timer to use UTC date timestamps.
• Updates bootstrap supervisord configuration file/priority to 20-redis-bootstrap.conf/20.
• Updates wrapper supervisord configuration file/priority to 50-redis-wrapper.conf/50.
• Fixes binary paths in systemd unit files; missing changes from last release.
• Fixes docker host connection status check in Makefile.
• Adds missing instruction step to docker-compose example configuration file.
• Adds inspect, reload and top Makefile targets.
• Adds improved clean Makefile target; includes exited containers and dangling images.
• Adds lock/state file to wrapper script.
• Adds SYSTEM_TIMEZONE handling to Makefile, scmi, systemd unit and docker-compose templates.
• Adds system time zone validation to healthcheck.
• Removes support for long image tags (i.e. centos-7-3.x.x).
• Removes REDIS_AUTOSTART_REDIS_BOOTSTRAP, replaced with ENABLE_REDIS_BOOTSTRAP.
• Removes REDIS_AUTOSTART_REDIS_WRAPPER, replaced with ENABLE_REDIS_WRAPPER.