Prior to this change, plugin config files are only allowed
to be placed in the collective config dir etc/plugins.
In order to support external plugin's drop-in deployment,
this commit made emqx_plugins module to read conf file
in application's etc dir
And move emqx app back to root level
After realising the challenges of developing plugins in standalone
rebar projects. it is perhaps more smooth to keep using emqx as
a rebar dependency
Shawn