Set default georchestra datadir location

[edevosc2c]

The purpose of this PR is to set the default geOrchestra datadir location. In order to avoid having to specify it in all our deployments.

/etc/georchestra is a standard directory used in Docker and even in Debian package for geOrchestra.

It is already included inside many of our Dockerfile: https://github.com/search?q=repo%3Ageorchestra%2Fgeorchestra%20%2Fetc%2Fgeorchestra&type=code and even sometimes in the application.yml itself: https://github.com/georchestra/georchestra/blob/5f9743f0a773eaf5f276ca459c80423e16cad364/datafeeder/src/main/resources/application.yml#L108

I'm proposing this PR because as of right now we can't customize the Dockerfile so we can't set a default geOrchestra datadir location through this file.

@pmauduit told me that you can still change the parameter even though it is compiled from the source code: https://docs.spring.io/spring-boot/reference/features/external-config.html. Using a method like JAVA_TOOL_OPTIONS: "-Dgeorchestra.datadir=/etc/georchestra". So nobody is stuck with this static directory.

[pmauduit]

Using a method like JAVA_TOOL_OPTIONS: "-Dgeorchestra.datadir=/etc/georchestra". So nobody is stuck with this static directory.

from my understanding of the official spring boot documentation I pointed, indeed. I have not tested though.
Please also note that using an environment variable GEORCHESTRA_DATADIR should also override the default behaviour (if it takes precedence over the yaml file from the classpath). And maybe doing so is less intrusive than having to define / override the JAVA_TOOL_OPTION one.

[pmauduit]

I don't really mind the change, but since we already do the same onto the default datafeeder configuration, then I am okay with it.