Change default value of docker image tags

Currently, values.yaml specifies master as the default value of image.tag and initImage.tag. However, installing the chart with those values will lead to problems for a longer-lived deployment: at some point the interface between the chart and the image (for example the set of ansible variables that are set through the ansibleSecrets helm value) will change, and if the wordpress pod is then restarted for any reason, it will download the new image which will fail to run.

One way to deal with this is to

have a version number;
promise to not change the image interface in minor versions, or only in a backwards-compatible way – that is, allow an older deployment to use an image with higher minor version;
push docker image tags for major versions;
set the default image.tag and initImage.tag to the current major version, and keep it updated through a release checklist.