ElasticsearchIllegalStateException when invoking _cat plugins

Reproducible using ES 1.1.0.

`/_cat/plugins` works fine when no plugins are installed.

But once a plugin is installed (eg. latest marvel), it throws an exception indicating that the resulting table constructed has 7 fields in the header row but not enough fields in the data rows to make it a valid table.

```
{
  "status" : 500,
  "error" : "ElasticsearchIllegalStateException[mismatch on number of cells 6 in a row compared to header 7]"
}
```

This issue has been introduced when we removed `isolation` feature in 1.1 branch. See c34d8bd.

Closes #5715

(cherry picked from commit abc453daf368225c4dde8d7d0fb0acdda8644f5f)