restapi: Add "size" support to templates

This patch adds support for new "size" detail specifications for the
templates collection.

When the "size" detail is requested the result will look like thiss:

  GET /templates;detail=+size HTTP/1.1
  All-Content: true

  <templates>
    <size>123</size>
    <template id="..." href="...">
      ...
    </template>
    ...
  </templates>

The "main" detail is intended to suppress output of the actual data, so that
only the size will be populated:

  GET /templates;detail=-main+size HTTP/1.1
  All-Content: true

  <templates>
    <size>123</size>
  </templates>

Change-Id: I6f39413ea33319a21fd48018017fce8030ecbf14
Bug-Url: https://bugzilla.redhat.com/1132249
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>