Deal more gracefully with YAML errors in neighbordb

[advornic]

Specifically:

• hidden tabs
• "---" marker in the middle of the file

[advornic]

Release note:

YAML serialization errors are now exposed in ZTPS logs:

DEBUG: [controller:170] JPE14140273: running post_node                                                              
ERROR: [topology:83] JPE14140273: failed to load file: /usr/share/ztpserver/neighbordb                              
ERROR: [topology:116] JPE14140273: failed to load neighbordb:                                                       
expected a single document in the stream                                                                            
  in "", line 26, column 1:                                                                                 
    patterns:                                                                                                       
    ^                                                                                                               
but found another document                                                                                          
  in "", line 35, column 1:                                                                                 
    ---                                                                                                             
    ^                                                                                                               
DEBUG: [controller:182] JPE14140273: response to post_node: {'status': 400, 'body': '', 'content_type': 'text/html'}
s7056.lab.local - - [03/Nov/2014 21:05:33] "POST /nodes HTTP/1.1" 400 0