Move to overcommit.slice so it can see all the CPUs

Summary:
1] Change to fb_timer (for hm_data_plane)

The API exposes a way to set custom properties, but also manually sets slice. We can override slice by specifying it twice, but it's ugly -- the resulting file ends up looking like
```
Slice=system-timers-trickle_test_runner.slice
Slice=overcommit-timers-trickle_test_runner.slice
```
This makes it not set the first Slice= value if Slice is part of the overrides.

Differential Revision: D52097721

fbshipit-source-id: bf6440cd97255e884edd89710586a1f9339ebf5e

cookbooks/fb_timers/templates/default/service.erb

@@ -22,7 +22,7 @@ Type=oneshot
 <% if @conf['envfile'] %>
 EnvironmentFile=<%= @conf['envfile'] %>
 <% end %>
-<% if node['fb_timers']['enable_named_slices'] %>
+<% if node['fb_timers']['enable_named_slices'] and !@conf['service_options'].key?('Slice') %>
 Slice=system-timers-<%= @conf['name'] %>.slice
 <% end %>
 <% @conf['commands'].each do |command| %>