Set the Xms value when Tez computes the heap size for a container

Currently, we only set the Xmx value.

Another useful change would be to change the calculation to not use the fraction blindly. For large containers - .e.g 10GB - this ends up leaving 2G of the container unused.
Instead we can add another parameter which indicates the max amount of memory to leave outside the heap. Maybe a separate jira for this.