Fix ERTS path in ejabberdctl with rebar3 release

Rebar 2 expands {{erts-vsn}} to "erts-$vsn", Rebar 3 expands it to just
"$vsn".  Make sure `make rel` doesn't end up with a "$vsn" directory
next to "erts-$vsn" (which happened when using Rebar 3), and make sure
that ejabberdctl expects both "erl" and "epmd" to be installed below
"erts-$vsn" (which it didn't when using Rebar 3).

ejabberdctl.template

@@ -12,6 +12,7 @@ ERLANG_NODE=ejabberd@localhost
 # define default environment variables
 [ -z "$SCRIPT" ] && SCRIPT=$0
 SCRIPT_DIR="$(cd "$(dirname "$SCRIPT")" && pwd -P)"
+ERTS_VSN="{{erts_vsn}}"
 ERL="{{erl}}"
 IEX="{{bindir}}/iex"
 EPMD="{{epmd}}"

rebar.config

@@ -181,7 +181,7 @@
         {overlay, [{mkdir, "var/log/ejabberd"},
                    {mkdir, "var/lib/ejabberd"},
                    {mkdir, "etc/ejabberd"},
-                   {copy, "rel/files/erl", "\{\{erts_vsn\}\}/bin/erl"}, % in rebar2 this prepends erts-
+                   {copy, "rel/files/erl", "erts-\{\{erts_vsn\}\}/bin/erl"},
                    {template, "ejabberdctl.template", "bin/ejabberdctl"},
                    {copy, "inetrc", "etc/ejabberd/inetrc"},
                    {copy, "tools/captcha*.sh", "lib/ejabberd-\{\{release_version\}\}/priv/bin/"},

vars.config.in

@@ -51,9 +51,10 @@
 {release, true}.
 {release_dir, "${SCRIPT_DIR%/*}"}.
 {sysconfdir, "{{release_dir}}/etc"}.
+{erts_dir, "{{release_dir}}/erts-${ERTS_VSN#erts-}"}.
 {installuser, "@INSTALLUSER@"}.
-{erl, "{{release_dir}}/{{erts_vsn}}/bin/erl"}.
-{epmd, "{{release_dir}}/{{erts_vsn}}/bin/epmd"}.
+{erl, "{{erts_dir}}/bin/erl"}.
+{epmd, "{{erts_dir}}/bin/epmd"}.
 {localstatedir, "{{release_dir}}/var"}.
 {libdir, "{{release_dir}}/lib"}.
 {docdir, "{{release_dir}}/doc"}.