fixes for packaging regressions for 3.2.1

Fixes for a couple of regressions discovered while packaging riak-3.2.0: * When riak was started via systemctl, the .service unit file had `riak foreground` (blindly following an advice from riak launcher script generated by rebar3.18), which resulted in pipe files in /tmp/erl_pipes/riak@127.0.0.1 not being created as they should be for `riak attach` to work. Whether it is a bug or feature in relx that comes with rebar3.18, is a matter for rebar3 developers to comment or investigate; for the time being, [changing](https://github.com/basho/riak/commit/18fb795835f104907527d157be5e3ed294a313cd) the starting command (back) to `riak start` appears to be sufficient. * If riak launcher script finds an empty VMARGS_PATH, it extracts `node` from the default vm.args which will always be set to "riak@127.0.0.1", while the proper value, as specified in /etc/riak.conf, is to be found in generated.conf/vm.*.args. It's probably cuttlefish's task to take care of it by exporting VMARGS_PATH, but it doesn't currently do so, meaning we should [do it](https://github.com/basho/riak/commit/2c41a8443a37e1f36f5cbdea1d47769ef22392e2) ourselves. * An [optimisation](https://github.com/basho/riak/commit/35d5c3f42acc1e6a5fcb68eccfaa258a34897b0a) that allows packagers (me) to reuse previously fetched deps and avoid downloading 100s of megs every time `make package` is run (similar optimisations have already been included for deb and rpm flavors). * An update of vars.config for alpine that should have been included in #1114 but wasn't.