Use image arg when generating accounts

Let's pass image via --image argument instead of passing overrides
(image.http_image) values when generating accounts.yaml.

Change-Id: I4819a91fea6b125f60cc752b236824335301afac

tasks/generate-accounts.yaml

@@ -8,6 +8,7 @@
        --debug \
        -v \
        --create \
+       --image {{ url_cirros_image }} \
        --out ./etc/tempest_admin.conf
   args:
     executable: /bin/bash
@@ -23,9 +24,9 @@
        --debug \
        -v \
        --create \
+       --image {{ url_cirros_image }} \
        --out ./etc/tempest_admin.conf \
-       {{ additional_tempestconf_params }} \
-       image.http_image {{ url_cirros_image }}
+       {{ additional_tempestconf_params }}
   args:
     executable: /bin/bash
     chdir: "{{ refstack_client_source }}"