Plugin bitcoind_: use defaults for missing environment variables

Previously undefined environment variables were treated as "None"
instead of using the supplied defaults (e.g. for "rpchost" or
"rpcport").

plugins/currency/bitcoin/bitcoind_

@@ -174,6 +174,8 @@ def parse_conf(filename):
 def get_env_options(*vars):
     options = Options()
     for var in vars:
+        value = os.getenv(var)
+        if value is not None:
             options[var] = os.getenv(var)
     return options