Logstash YUM uninstaller fails when using LDAP 'logstash' account

Hi,

I accidentally installed Logstash 6.0.0~rc1-1 (YUM pre-release repo was left enabled by mistake after we were testing newer versions of Beats). When I went to remove logstash, I found that I couldn't as the uninstall script failed trying to remove the 'logstash' user and group because we have them in LDAP and not local to the server.

```
Running transaction
userdel: cannot remove entry 'logstash' from /etc/passwd
groupdel: cannot remove the primary group of user 'logstash'
error: %preun(logstash-1:6.0.0~rc1-1.noarch) scriptlet failed, exit status 8
Error in PREUN scriptlet in rpm package 1:logstash-6.0.0~rc1-1.noarch
```

Would it be feasible to update the preuninstall scriptlet to have something like:
```
if getent passwd logstash >/dev/null && grep '^logstash:' /etc/passwd > /dev/null ; then
userdel logstash
fi

if getent group logstash > /dev/null && grep '^logstash:' /etc/group > /dev/null ; then
groupdel logstash
fi
```
So that the uninstaller only attempts to remove the logstash user and group if they are local to the host that we're removing the package from?