Enhancement: Allow user to upload ZTPServer configuration files

[phil-dileo]

Right now, the packer build clones the ztpserver-demo files from Github, which is fine for a default action, but it would be nice if the user could pass into the build some local files that would be automatically placed into the correct directories.

The folder would use standard names, like:

• neighbordb
• nodes/
• ztpserver.conf
• bootstrap.conf
• ...