Check for mandatory environment variables
Check for mandatory environment variables

We added a `set -u` in previous commit which causes a "unbound variable"
failure if some environment variables weren't defined.

vrutkovs/mysql,mfojtik/mysql,bparees/mysql,hhorak/mysql-container,stevekuznetsov/mysql,vrutkovs/mysql,mfojtik/mysql,rhcarvalho/mysql,bparees/mysql,olysonek/mysql,hhorak/mysql-container,praiskup/openshift-mysql,rhcarvalho/mysql,openshift/mysql