Better healthcheck geoserver

[edevosc2c]

This change the healthcheck of geoserver because:

• The healthcheck fail when some external datastores doesn't work.
• This is compute expensive to load this request on large datastores.

Half downside:

• layer geor:public_layer needs to exist

[jeanpommier]

Thanks Emilien. I believe it was the healthcheck that was ised, historically, although I can't find it back.

It should be more efficient indeed, thanks for the change