Allow specification of own user name

[gingerwizard]

Allow the specification of a different username under which elasticsearch should be run. Currently we only support "elasticsearch".

I propose we do not create this user if it doesn't exist i.e. we assume its existence.

[gingerwizard]

@jakommo

[jakommo]

If we always use the deb/rpm the user should exist anyways, but it could be necessary to work around #36, where the UID/GID would need to be the same if NFS is used for snapshots and idmapd isn't in place.
If we decide to do it, we also must edit the tmpfiles.d files created for systemd, as they define the user for the /var/run/... dir.

[gingerwizard]

I dont think this is an issue. We already allow a username and group to be specified and we now support specification of the uid and gid.