Install wizard multisig improvement

Preserves independence of wizard.py from wallet type;
the dependence is expressed entirely through actions

Fixes #1611

lib/wallet.py

@@ -1817,6 +1817,10 @@ class Multisig_Wallet(BIP32_Wallet, Mnemonic):
     def get_master_public_keys(self):
         return self.master_public_keys
 
+    def get_action(self):
+        if not self.get_master_public_key():
+            return 'create_seed_and_cosigners'
+
 
 class OldWallet(Deterministic_Wallet):
     wallet_type = 'old'