Update NTP test logic to better handle set and get NTP servers
Update NTP test logic to better handle set and get NTP servers

Changes:
    - Added new keyword to wait for GET request.
    - Add test setup in the test to enabled NTP
    - Update documentation and variable string

Tested:

robot -v OPENBMC_HOST:xx.xx.xx.xx --include Verify_NTP_Server_Set
      --include Verify_NTP_Server_Setting_Persist_After_BMC_Reboot
      redfish/managers/test_managers_bmc_time.robot

Change-Id: I7976e4e8775b8f1e4b6b744bb54df386dc0d67fd
Signed-off-by: George Keishing <bef0a9ecac45fb57611777c8270153994e13fd2e@in.ibm.com>

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation