nodetool has all lib paths set up correctly by reading the
persisted RELEASES file
calling hocon escript directly has two problems:
1. it may not be able to find the beams compiled
2. if we use ERL_LIBS, it may find the version after hot beam upgrade
Prioro to this change, ekka lib dir was added by inspecting
the ekka lib version in persisted RELEASES file.
However, this file may not be the true version if a hot
upgrade failed in the middle.
In this fix, it reads the per-version emqx.rel file
content to load lib paths.
Zaiming Shi