Plugins need a temp dir kibana user can write to

[LeeDr]

Kibana needs a temp dir (owned by kibana user) for plugins to write to.

I'll make a separate issue for the reporting plugin in x-plugins to USE that temp dir.

This is the ownership of the dirs in kibana on Ubuntu after a 5.0.0-alpha2 install;

root@U14K:~/kibana# ls -l /opt/kibana/
total 44
drwxr-xr-x  2 root   root 4096 May  9 14:03 bin
drwxrwxr-x  2 root   root 4096 May  9 14:09 config
drwxrwxr-x  4 root   root 4096 May  9 14:07 installedPlugins
-rw-rw-r--  1 root   root  563 May  3 12:11 LICENSE.txt
drwxrwxr-x  6 root   root 4096 May  9 14:03 node
drwxrwxr-x 88 root   root 4096 May  9 14:03 node_modules
drwxrwxr-x  3 kibana root 4096 May  9 14:03 optimize
-rw-rw-r--  1 root   root  708 May  3 12:11 package.json
-rw-rw-r--  1 root   root 2446 May  3 12:11 README.txt
drwxr-xr-x  9 root   root 4096 May  9 14:03 src
drwxr-xr-x  2 root   root 4096 May  9 14:03 webpackShims

[w33ble]

#7275 is a consideration, as anything we create will need to work in the os packages as well.

#6674 fixes some permissions on optimize, and will have to be done with whatever target path we create here as well.

According to elastic/logstash#5101, Logstash has a state/directory thing they've had to manage and move around as well. Probably worth chatting with some of them to see what they are storing and how they handle it.