Prior to this change, the template file etc/emqx.conf is used
directly in testing, as a result, mustache style directories
are created e.g. `{{ platform_log_dir }}` which should have been
replaced with a config varialbe e.g. `log`
In this change, Makefile targets are added as `ct` dependency
to download bbmustach, load the template input, replace with
variableds defined in 'vars' file, finally to etc/gen.emqx.conf.
The direct usage of etc/emqx.conf in test code are replaced with
gen.emqx.conf
turtleDeng