Move systemd files from /etc to /usr/lib

As documented in systemd's manual pages tmpfiles.d(5) and systemd.unit(5),
a package should install its default configuration in /usr/lib, which can
be overriden by system administrators in /etc.

New locations in the rpm:
/usr/lib/systemd/system/elasticsearch.service
/usr/lib/tmpfiles.d/elasticsearch.conf