Fix ntp sync

Use default ntp server as in salt-formula-ntp

Change-Id: I40ea47fea89a2dcfc099854783b3c88386fc0394
diff --git a/tcp_tests/managers/saltmanager.py b/tcp_tests/managers/saltmanager.py
index b2abdce..272621a 100644
--- a/tcp_tests/managers/saltmanager.py
+++ b/tcp_tests/managers/saltmanager.py
@@ -298,7 +298,8 @@
                    'fi;'
                    'sleep 3;'
                    # note: maas-rackd will return 'pool' after start
-                   'sed -i \'s/^pool /server /g\' /etc/ntp/maas.conf;'
+                   'sed -i \'s/^pool ntp.ubuntu.com/server ntp.cesnet.cz/g\' '
+                   '/etc/ntp/maas.conf;'
                    'if [ -x /usr/sbin/ntpdate ]; then'
                    '  ntpdate -s ntp.ubuntu.com;'
                    'else'