chronyd keeps running even if unit file disabled from systemd startup

Issue

• System has both NTP and chrony packages: We need to disable chrony and enable ntp without uninstalling chrony package.
• Disabled chrony Unit file from systemd startup but still it shows started and enabled after executing following timedatectl command.

# timedatectl set-ntp true

# timedatectl 
      Local time: Thu 2017-07-13 03:02:29 IST
  Universal time: Wed 2017-07-12 21:32:29 UTC
        RTC time: Wed 2017-07-12 21:32:28
       Time zone: Asia/Kolkata (IST, +0530)
     NTP enabled: yes
NTP synchronized: no
 RTC in local TZ: no
      DST active: n/a