[FIX] contract: Process invoices limit in a lower method

To avoid blocking the queue when there are more than the specified limit
of contracts, we process the limit while creating invoices instead
of while searching for contracts, and break the process when the
max of invoices has been created.

See https://github.com/OCA/contract/pull/260#pullrequestreview-192187022
for more details.

In case you need to use this new feature in the cron, it is
also modified as `noupdate=1`.

contract/data/contract_cron.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding='UTF-8'?>
-<odoo>
+<odoo noupdate="1">
 
     <record model="ir.cron" id="account_analytic_cron_for_invoice">
         <field name="name">Generate Recurring Invoices from Contracts</field>

contract/models/account_analytic_account.py

@@ -260,13 +260,18 @@ class AccountAnalyticAccount(models.Model):
         return invoice
 
     @api.multi
-    def recurring_create_invoice(self):
+    def recurring_create_invoice(self, limit=None):
         """Create invoices from contracts
 
+        :param int limit:
+            Max of invoices to create.
+
         :return: invoices created
         """
         invoices = self.env['account.invoice']
         for contract in self:
+            if limit and len(invoices) >= limit:
+                break
             ref_date = contract.recurring_next_date or fields.Date.today()
             if (contract.date_start > ref_date or
                     contract.date_end and contract.date_end < ref_date):
@@ -302,8 +307,8 @@ class AccountAnalyticAccount(models.Model):
             '|',
             ('date_end', '=', False),
             ('date_end', '>=', today),
-        ], limit=limit)
-        return contracts.recurring_create_invoice()
+        ])
+        return contracts.recurring_create_invoice(limit)
 
     @api.multi
     def action_contract_send(self):