added reconnect dealy for the minion zmq-pull socket

If the saltmaster is stopped or restarted and the sockets are reopened, the minions try to reconnect immediately to the publish-socket of the saltmaster. The default delay for this reconnect is 100ms (which is actually a ZeroMQ value and not specific to salt).

In large enviroments this can lead to a syn-flood on port 4505 on the master.

This patch adds the possibility to configure the minions pull-socket to use a random delay between ZMQ_RECONNECT_IVL and ZMQ_RECONNET_IVL_MAX. I added a more detailed explanation to the master configfile to explain what this does. If the settings are commented, the default behaviour of the minion is not changed.