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
* Pin cuttlefish rebar3 branch
* No shallow clone for rebar3
* Replace timer:sleep with receive after for deterministic
* Build with only rebar3
* bbmustache as a test dependency
* Change to plain deps
* Use shallow clone by default
* Add ct-helpers as test dep