Sync first stage config with v2021dev3

All the release channels now use the same first stage based on the v2020
NFS artifacts rather than the old ones based on 18.12.

This means that tests are currently broken because LAVA fails to detect
the login prompt as the string changed.

Update the config using what is being used by v2021dev3.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

templates/common-boot-tpl.yaml

@@ -68,14 +68,14 @@ actions:
       method: {{boot_method}}
       commands: nfs
       prompts:
-        - 'nfsuser@apertis:'
+        - 'user@apertis:'
         - '\$ '
         - '/home/user # '
       auto_login:
-        login_prompt: 'apertis-nfs login:'
-        username: nfsuser
+        login_prompt: 'apertis login:'
+        username: user
         password_prompt: 'Password:'
-        password: nfsuser
+        password: user
         login_commands:
           - sudo su