Handle NTPServers list per the specification

The Redfish specification for PATCH of arrays defines a number of
requirements.
- Setting a value to null, should remove it from the list.
- Setting a value to empty object "{}" should leave the value unmodified
- Values at indexes larger than whats included in the PATCH request
  shall be removed.

This commit attempts to fix this behavior for NTPServers and make it
correct.  It does this by first getting the list of NTP servers, then
walking the list in parallel with the list given in the patch, and
either modifying or changing the list as the spec requires before
setting the setting across the system.

It also turns out that the current behavior of unpacking nlohmann::json
objects requires an object to be an array, object, or null, which
doesn't allow unpacking the strings required in this case, so that check
is removed.  A quick inspection shows that we don't unpack nlohmann
objects very often, and this should have no impact.

Tested:
Redfish-protocol-validator tests for NTPServers now pass

'''
curl -vvvv --insecure --user root:0penBmc https://192.168.7.2/redfish/v1/Managers/bmc/NetworkProtocol

 -X PATCH -d '{"NTP": {"NTPServers": []}}'
'''
Used to patch values succeeds with various "good" values;
["time-a-b.nist.gov", "time-b-b.nist.gov"]
[{}, {}]
["time-a-b.nist.gov", null]
[]

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I23a8febde34817bb0b934e46e2b77ff391b52a57