Do not allow edit of variable rate for tier in fixed rate_detail

This Simplify rates editor. The rates editor is a complex form and
user can hardly comprehend what it does. By removing useless field we
give at least some clue to the user.

Now, let's discuss, why this is valid change:
 - For variable rate the computation goes like
   cost = fixed_rate + variable_rate * metric
 - For fixed rate the computation goes like
   metric = 1
   cost = fixed_rate + variable_rate * metric
(transferred from ManageIQ/manageiq@94bebcd)

app/views/chargeback/_cb_tier_edit_values.html.haml

@@ -1,4 +1,5 @@
 - tier = @edit[:new][:tiers][detail_index][row_within_rate]
+- detail = @edit[:new][:details][detail_index]
 %td
   - tier_val = (tier[:start].to_s.eql? 'Infinity') ? '' : tier[:start]
   = text_field_tag("start_#{detail_index}_#{row_within_rate}", tier_val,
@@ -11,5 +12,9 @@
   = text_field_tag("fixed_rate_#{detail_index}_#{row_within_rate}", tier[:fixed_rate],
     :maxlength => MAX_NAME_LEN, 'data-miq_observe' => {:interval => '.5', :url => url}.to_json)
 %td{:align => 'right'}
-  = text_field_tag("variable_rate_#{detail_index}_#{row_within_rate}", tier[:variable_rate],
-    :maxlength => MAX_NAME_LEN, 'data-miq_observe' => {:interval => '.5', :url => url}.to_json)
+  - if detail['group'] == 'fixed' && (tier[:variable_rate].blank? || tier[:variable_rate].zero?)
+    /simplify editor, by disabling variable rate edit for fixed rates (allow edit only when this value already exists)
+    = '-'
+  - else
+    = text_field_tag("variable_rate_#{detail_index}_#{row_within_rate}", tier[:variable_rate],
+      :maxlength => MAX_NAME_LEN, 'data-miq_observe' => {:interval => '.5', :url => url}.to_json)