Allow helm chart threddsConfig override and runAs arbitrary uid

[bstrdsmkr]

Some kubernetes environments require running services as arbitrary UIDs. The Thredds image attempts to copy files to /opt/tomcat/content/thredds on startup which fails in these environments.

This PR mounts an emptyDir at /opt/tomcat/content/thredds to allow the startup procedure to write it's files. As a side effect, the catalog.xml and threddsConfig.xml baked into the image are overwritten with the empty directory. catalog.xml was already being mounted from a configMap so that hasn't changed, but now threddsConfig.xml is also mounted from a separate configMap.

Both configMaps default to the same existing documents, but can now be overridden at deploy time with something like --set-file data.thredds.config."catalog\.xml"=/path/to/local/catalog.xml