Ensure elasticsearch "plugins" directory exists on startup

[GUI]

Since upgrading Elasticsearch to v2 (#315), it looks like the "plugins" directory has to exist, or Elasticsearch will fail to start. We should add this to the various other directories we ensure are created on startup.

Interestingly, this hadn't cropped up until I built a prerelease RPM package from master. So I think this directory might exist during the normal build process, but I suspect it's an empty directory and might not be getting included in the binary packages.