Add env variable on Server Installation

Following:

https://github.com/DIRACGrid/DIRAC/issues/7183

It's good to set:

export MYSQL_CONNECTION_GRACE_TIME=0

in the /opt/dirac/bashrc

on the servers. (I don't think it's needed on the client).

This setting should go both in the doc and in the deployment scripts (even if in DIRAC future release it should become the default).