Sentinel: fix slave promotion timeout.

If we can't reconfigure a slave in time during failover, go forward as
anyway the slave will be fixed by Sentinels in the future, once they
detect it is misconfigured.

Otherwise a failover in progress may never terminate if for some reason
the slave is uncapable to sync with the master while at the same time
it is not disconnected.
Sentinel: SENTINEL_SLAVE_RECONF_RETRY_PERIOD -> RECONF_TIMEOUT

Rename define to match the new meaning.
Sentinel test: provide basic commands to access instances.
Prefix test file names with numbers to force exec order.
Sentinel test: info fields, master-slave setup, fixes.
Sentinel test: basic tests for MONITOR and auto-discovery.
Sentinel test: basic failover tested. Framework improvements.
Sentinel test: check reconfig of slaves and old master.