Give desktop_browser 4G of RAM

We're seeing a lot of failures recently in desktop_browser caused
by the browser or desktop getting OOM killed. I've filed a bug
on this: https://bugzilla.redhat.com/show_bug.cgi?id=2133829
but we need to try and work around it, having the test failing
constantly is no good. Let's see if 4G of RAM is enough.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

templates.fif.json

@@ -966,6 +966,7 @@
                 "BOOTFROM": "c",
                 "HDD_1": "disk_%FLAVOR%_%MACHINE%.qcow2",
                 "POSTINSTALL": "desktop_browser",
+                "QEMURAM": "4096",
                 "START_AFTER_TEST": "%DEPLOY_UPLOAD_TEST%"
             }
         },