Description of problem:

The available fields in reports are computed once *per report type*:
https://github.com/ManageIQ/manageiq/blob/a3e4523b822f8cc0effaf58ff5296f77f2bd158f/lib/miq_expression.rb#L1193-L1207

- New tag categories will not appear in report editor.
- New custom attribute types will not appear in report editor.

They will appear if you select a new report type you didn't use before but then they're remembered forever — until server restart (probably UI worker).

Version-Release number of selected component (if applicable):
5.7.0.9-beta2.1.20161101182054_eb0afaa

How reproducible:
always

Steps to Reproduce:
1. Open Report Editor: Cloud Intel -> Reports -> Reports accordion -> 
   Configuration -> Add a new report
   and choose some "Base the report on" type e.g. VMs and Instances
2. Cancel.  (just populating the dialog caches the fields)
3. Create new tags category:
   top right EVM menu -> Configuration -> Region 0 in explorer -> 
   My Company Categories tab -> "<click this row to create>"
   Let's say you called it "AAAA"
4. Create an entry under the category:
   My Company Tags tab -> choose the new "AAAA" category from dropdown -> "<click this row to create>"
5. Open Report Editor again, and choose same "Base the report on" type as in (1)

Actual results:
The "Available Fields" box does not contain the new "My Company Tags : AAAA"

Expected results:
The "Available Fields" box should contain "My Company Tags : AAAA"


Additional info:
https://github.com/ManageIQ/manageiq/issues/12465