Rudimentary support for json format juniper get_config

If no format attribute is specified get_config returns config as a plain text, but in json is specified, it returns a dict. 
Probably it still needs further polishing. e.g I cast it to a str, but maybe i could stay as dict.  It's my first pull request ever, Sorry if I messed something up.

napalm/junos/junos.py

@@ -2459,12 +2459,17 @@ def get_config(self, retrieve="all", full=False, sanitized=False, format="text")
         }
         if retrieve in ("candidate", "all"):
             config = self.device.rpc.get_config(filter_xml=None, options=options)
-            rv["candidate"] = str(config.text)
+            if options["format"] == "text":
+                rv["candidate"] = str(config.text)
+            else:
+                return str(config["configuration"])
         if retrieve in ("running", "all"):
             options["database"] = "committed"
             config = self.device.rpc.get_config(filter_xml=None, options=options)
-            rv["running"] = str(config.text)
-
+            if options["format"] == "text":
+                rv["candidate"] = str(config.text)
+            else:
+                return str(config["configuration"])
         if sanitized:
             return napalm.base.helpers.sanitize_configs(rv, sanitize_strings)