Revert "Ubuntu: deploy new AMI to eu-central-1 region"

This reverts commit 2b038ab04cd6e635b7d63c463338d05c149608bc.

Reason for revert: <Can not build AMI for eu-central-1 region. https://ci.staging.trustedfirmware.org/job/mbedtls-amis/28/console>

Change-Id: I9762a7aed27bd94afda8b80d80c855d630976169
diff --git a/packer_ubuntu.json b/packer_ubuntu.json
index e5eefe2..43c09ba 100644
--- a/packer_ubuntu.json
+++ b/packer_ubuntu.json
@@ -1,6 +1,6 @@
 {
   "variables": {
-    "vpc_region": "eu-central-1",
+    "vpc_region": "us-east-1",
     "ssh_username": "ubuntu"
   },
   "builders": [