Show warning when append key id to NTP server and when deleting a key which is bound to an NTP server

The Jira ticket is:

https://arraynetworks.atlassian.net/browse/TWSD-1081
After discussing with QA,

the ticket sould contain these enhancement:

1. The message of empty key should be "key is mandatory for server" or something like that

2. If the key is not exist, we should show message when bind server with key

3. If the key is bound to a server, we should show warning when delete the key
So I followed the requirement and changed the code:

I added function to check is the Key ID exist,

and added a function to check is the Key is bound to an NTP server.

Then write the relative condition and show warning messages.
You can see the attachment for CLI testing result.