In production, the sending of emails by using Internet Message Posts created activity failures : 

```
Node: activities-XX
Failures: 1
User name: 'System Processes'
Uid: 1234567890
Document: /erp5/person_module/XXXX
Method: Entity_sendEmail
Arguments: ()
Named Parameters: {'cc_url': None, 'from_url': None, 'event_relative_url': 'event_module/YYYY'}

Exception:
  Module Products.CMFActivity.Activity.SQLBase, line 607, in dequeueMessage
    transaction.commit()
  Module transaction._manager, line 123, in commit
    return self.get().commit()
  Module transaction._transaction, line 280, in commit
    reraise(t, v, tb)
  Module transaction._transaction, line 271, in commit
    self._commitResources()
  Module Products.TIDStorage.transaction_transaction, line 261, in _commitResources
    result = original__commitResources(self, *args, **kw)
  Module transaction._transaction, line 416, in _commitResources
    reraise(t, v, tb)
  Module transaction._transaction, line 393, in _commitResources
    rm.tpc_vote(self)
  Module ZODB.Connection, line 797, in tpc_vote
    s = vote(transaction)
  Module ZEO.ClientStorage, line 1072, in tpc_vote
    return self._check_serials()
  Module ZEO.ClientStorage, line 902, in _check_serials
    raise s
ReadConflictError: database read conflict error (oid 0x12345 serial this txn started with 0x12345 2018-05-16 06:27:17.471887, serial currently committed 0x12345 2018-05-16 06:53:10.977183)
``` 

This Merge Request proposes to allow Entity_sendEmail to be retried in case of ConflictError, as now Entity_sendEmail is doing more than what it used to be.

For this, we need to isolate the call to sendMailHostMessage to make sur that email won't be sent several times if the activity "Entity_sendEmail" is retried.

We also make sure that the activity created to send a message through MailHost gets the correct parameters to be never retried (retry_max=0 and retry_conflict=False)

/reviewed-on nexedi/erp5!672

But only one mail will be sent out by MailHost

Reduce the number of acquired method calls by grouping them.

See 382a3ae2

- hardcoded parameter removed
- minor changes in slapos master communicator

This reverts commit 3a811c0e.

This reverts commit 02e06ea3.

/reviewed-on nexedi/erp5!671

- code for generateConfigurationList moved to scalability distributor
- general code refactoring and cleanup

As since the introduction of Internet Message Post it is more likely that
conflicts happen. Also, this operation is now safe as sending email in
Entity_sendEmail is made transactionally (as the interaction with MailHost
is done through the creation of an activity)

Also, do not try to reprocess the activity if something goes wrong,
as it may spam the user.

1. accept a list of converison server
2. retry in case of network error

/reviewed-on !670

/reviewed-on nexedi/erp5!669