Make rpm/deb packages more similar to Elasticsearch

[epixa]

Reference: https://www.elastic.co/guide/en/elasticsearch/reference/master/rpm.html#rpm-layout

We currently install to /opt, where Elasticsearch uses /usr, /var, et al. Let's move towards Elasticsearch's model.

Major tasks:

• Make all relevant paths set via a build-specific config
• Update ospackages task to use file layout similar to Elasticsearch
• Write tool for migrating stored plugin state to the new location (e.g. phantomjs in reporting)

[untergeek]

Recommend looking over the changes in elastic/logstash#5341

[jbudz]

@epixa @BigFunger elasticsearch has /usr/share/elasticsearch/plugins, our equivalent folder being kibana/installedPlugins. Do/can we rename this to plugins?

[BigFunger]

The decision to use `kibana/installedPlugins' was made during the creation
of the plugin system, and the plugin installer just used that. @spalger,
can you provide some info on the ramifications of changing that?

[epixa]

It's possible we were just trying to avoid ambiguity between non-core plugins and core plugins (which are located at src/plugins.

For consistency's sake, we could rename installedPlugins to plugins and then rename src/plugins to something like src/core (suggestions welcome).

[jbudz]

#6728 would be useful too.

[jbudz]

Closing this out, #7370 for docs
#7308, #7440, #7457, #7562 all had parts of this.