Replace ntp package from ntpd to chrony to support NTS
Support secure ntp feature 
Support NTP md5 key

AN(config)#show ntp status

Chrony daemon is running.
server time.cloudflare.com iburst nts

server 172.20.0.119 iburst key 10

server 1.2.3.40

server 1.2.3.222
Chrony Sources:

MS Name/IP address         Stratum Poll Reach LastRx Last sample

===============================================================================

^- time.cloudflare.com           3   6     7     1  -5997us[-5997us] +/-   51ms

^* 172.20.0.119                  2   6    17     2  +2764us[+3037us] +/- 5548us

^? 1.2.3.40                      0   6     0     -     +0ns[   +0ns] +/-    0ns

^? 1.2.3.222                     0   6     0     -     +0ns[   +0ns] +/-    0ns
Chrony Authentication Data:

Name/IP address             Mode KeyID Type KLen Last Atmp  NAK Cook CLen

=========================================================================

time.cloudflare.com          NTS     1   30  128    4    0    0    8   64

172.20.0.119                  SK    10    1   56    -    0    0    0    0

1.2.3.40                       -     0    0    0    -    0    0    0    0

1.2.3.222                      -     0    0    0    -    0    0    0    0