AUM OTA manual: add proxy server precondition

Add the description of how to add the proxy address for OSTree allowing online upgrades both for ostree and AUM in restricted networks.

Signed-off-by: Denis Pynkin denis.pynkin@collabora.com

test-cases/apertis-update-manager-ota-api.yaml

@@ -20,6 +20,10 @@ metadata:
       propagated to the hardware clock so it is stay set on the next reboot if
       the power is not plugged off.
     - $ sudo timedatectl --adjust-system-clock set-time "2019-08-21 18:49:03" # use the appropriate date
+    - >
+      If direct access to repository with updates for DUT is restricted and proxy server should be used,
+      then need to add the address of this proxy for OSTree on DUT by command:
+    - $ sudo ostree config set 'remote "origin"'.proxy "http://10.168.128.45:3128"
 
   resources:
     - "The DUT u-boot environment must be clean: in u-boot, run: `env default -a` followed by `saveenv`"