metal3: Rename DISTRIBUTION -> IMAGE_OS

IMAGE_OS is the variable that is used when running the tests, so to
avoid unnecessary variables we want the same variable name here also.

Change-Id: I9ddb64414a142ebd78550c4546dec671f0f791a7
diff --git a/jjb/metal3/projects.yml b/jjb/metal3/projects.yml
index 03e54df..070c399 100644
--- a/jjb/metal3/projects.yml
+++ b/jjb/metal3/projects.yml
@@ -58,7 +58,7 @@
     ci_github_repo: '${{ghprbAuthorRepoGitUrl}}'
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a5_integration_tests_centos
@@ -90,7 +90,7 @@
       name: "baremetal-operator"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a5_integration_tests_centos
@@ -110,7 +110,7 @@
       name: "ip-address-manager"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a5_integration_tests_centos
@@ -126,7 +126,7 @@
     name: "metal3_dev_env"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a4_main_integration_tests_centos
@@ -163,7 +163,7 @@
       name: "cluster-api-provider-metal3"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a5_integration_tests_centos
@@ -192,7 +192,7 @@
       name: "ironic-image"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution:
+        image_os:
           - ubuntu
           - centos
     - capm3_v1a5_integration_tests_centos
@@ -211,7 +211,7 @@
       name: "ironic-ipa-downloader"
     jobs:
     - capm3_v1a4_integration_tests:
-        distribution: ubuntu
+        image_os: ubuntu
     - capm3_v1a5_integration_tests_ubuntu
     - capm3_v1b1_integration_tests_centos
     - capm3_v1b1_integration_tests_ubuntu