Use serial number instead of system MAC as the unique system ID

This pull request makes the unique node identifier configurable at a global level.  The two supported options are 'systemmac' or 'serialnumber'.   To configure which one to use edit the ztpserver.conf file and configure the identifier.

```
[default]
identifier = systemmac
# identififer = serialnumber
```

__Note: this is a global setting that affects all nodes in the system including nodes that have already been provisioned__


Release note:

Any node that was previously configured using one identifier will no longer be recognized unless the entry for the node is manually modified.  Do not change this value on a production system without first understanding the consequence of this change