Fix libnss-mysql install

On recent setup refactoring to better support backend/frontend
deployments, we introduced variables to manage DB server location

This patch set a default host and distinguish case where an empty
hostname was used to detect if we need to bootstrap the db (local
deployments).

Change-Id: I3db4660dd36798f8e12a103108c0f9a5e306881e
Merge commit 'refs/changes/78/578/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix libnss-mysql install

Change-Id: Ieaf1e52f99534a803eaa3fbea2c96ab76df8ab0e
This is Tuleap 6.2.99.6

Change-Id: I2526e47f22fafb291742466fd5a572d2e31ed01f