Updates the default value of 'monasca_ntp_server' from
'external_ntp_servers[0]' to '0.pool.ntp.org'. This is due to the
removal of the 'external_ntp_servers' variable as part of the removal of
Chrony deployment.
Change-Id: I2e7538a2e95c7b8e9280eb051ee634b4313db129
