switch test-ci to Mirantis ubuntu mirror

Change-Id: I13912cac77870eab5e1d930e2a11a14bb01ec1ff
Bug-related: PROD-26050 (PROD:26050)
PROD: https://mirantis.jira.com/browse/PROD-26050
diff --git a/test-salt-formulas-pipeline.groovy b/test-salt-formulas-pipeline.groovy
index 52c7d79..404c0d0 100644
--- a/test-salt-formulas-pipeline.groovy
+++ b/test-salt-formulas-pipeline.groovy
@@ -106,8 +106,8 @@
               img.inside("-v ${env.WORKSPACE}/:/formula/ -u root:root --cpus=4 --ulimit nofile=4096:8192") {
                 sh('''#!/bin/bash -xe
                       cd /etc/apt/
-                      echo "deb [arch=amd64] http://cz.archive.ubuntu.com/ubuntu xenial main restricted universe" > sources.list
-                      echo "deb [arch=amd64] http://cz.archive.ubuntu.com/ubuntu xenial-updates main restricted universe" >> sources.list
+                      echo "deb [arch=amd64] http://mirror.mirantis.com/nightly/ubuntu xenial main restricted universe" > sources.list
+                      echo "deb [arch=amd64] http://mirror.mirantis.com/nightly/ubuntu xenial-updates main restricted universe" >> sources.list
                       echo 'Acquire::Languages "none";' > apt.conf.d/docker-no-languages
                       echo 'Acquire::GzipIndexes "true"; Acquire::CompressionTypes::Order:: "gz";' > apt.conf.d/docker-gzip-indexes
                       echo 'APT::Get::Install-Recommends "false"; APT::Get::Install-Suggests "false";' > apt.conf.d/docker-recommends