Add environment variables processing for maxscale.cnf

For Docker users (and maybe for other scenarios) it would be great if MaxScale substituted values for shell environment variables in the configuration file before reading the values. So that in case like this:

[node1]

type=server

address=$MARIADB_IP1

port=6603

protocol=MySQLBackend

[node2]

type=server

address=$MARIADB_IP2

port=6603

protocol=MySQLBackend

we get easy and clean way to pass IP addresses of back end servers,

Workarounds (like running script to pre-process template configuration files or passing configuration file from the host environment exist. but the problem is quite common to get some documented solution out of the box.

It may be a modification to startup script, or adding more predefined variables like MAXSCALE_OPTIONS, or just change in the code for options parsing.