Enable leafrefs in ietf-alarms

[dpino]

When YANG leafref datatype was not supported, all leafrefs in
ietf-alarms were commented and used string as type. The patch undoes
that change, resetting ietf-alarms to its original state.

[dpino]

To test it I informally run a lwaftr.conf that raises one alarm and query it with alarms get-state.

...
  alarm-list {                              
    alarm {                                 
      alarm-type-id arp-resolution;         
      alarm-type-qualifier '';              
      resource 32684;                       
      alarm-text "Make sure you can resolve external-interface.next-hop.ip address manually.  If it cannot be resolved, consider setting the MAC address of the next-hop directly.  To do it so, set external-interface.next-hop.mac to the value of the MAC address.";
      is-cleared false;                     
      last-changed 2018-06-13T13:55:49Z;    
      perceived-severity critical;          
      status-change {                       
        time 2018-06-13T13:55:49Z;          
        alarm-text "Make sure you can resolve external-interface.next-hop.ip address manually.  If it cannot be resolved, consider setting the MAC address of the next-hop directly.  To do it so, set external-interface.next-hop.mac to the value of the MAC address.";
        perceived-severity critical;        
      }                                     
      time-created 2018-06-13T13:55:49Z;    
    }                                       
    last-changed 2018-06-13T13:55:49Z;      
    number-of-alarms 1;                     
  }                                         
  summary {                                 
    alarm-summary {                         
      cleared 0;                            
      cleared-closed 0;                     
      cleared-not-closed 0;                 
      not-cleared-closed 0;                 
      not-cleared-not-closed 1;             
      total 1;                              
    }                                       
  }