Use different pidfile for builder daemon

Because we may want to colocate the nodepoold and nodepool builder
daemons they need to use different pidfiles. Make that the case by
default.

Change-Id: I563e59855f70c3e28cc1daff8bd0a9d063e179c0

nodepool/cmd/builder.py

@@ -41,7 +41,8 @@ class NodePoolBuilder(nodepool.cmd.NodepoolApp):
                             help='path to log config file')
         parser.add_argument('-p', dest='pidfile',
                             help='path to pid file',
-                            default='/var/run/nodepool/nodepool.pid')
+                            default='/var/run/nodepool-builder/'
+                                    'nodepool-builder.pid')
         parser.add_argument('-d', dest='nodaemon', action='store_true',
                             help='do not run as a daemon')
         self.args = parser.parse_args()