Add a directive to allow users to configure system variables in Bamboo

From BSP-1168 -

Please add the following lines to the Bamboo startup script for Unix (the bamboo.sh bash script):

if [ -f /etc/sysconfig/bamboo ]; then
. /etc/sysconfig/bamboo
fi

That will allow us to add environment customizations, eg. setting environment variables, to the /etc/sysconfig/bamboo file.

Thanks!