CLUSTER RESET: Flush dataset if node is a slave.

For non-empty masters, CLUSTER RESET is denied, and the user requires to
start to reset a node by explicitly clearing it with FLUSHALL.
However CLUSTER RESET when executed with slaves don't have this
restrictions since data is just a replica of the master, and with
read-only slaves it is also not possible to remove the data set. However
the node was turned from slave to master after a reset, without touching
the old slave data. This is 99.99% of times not appropriate and forces
full resets to follow this path to work with both slave and master
nodes:

    FLUSHALL
    CLUSTER RESET HARD
    FLUSHALL

Since we need the first flushall for masters, and the second for slaves.

This commit changes the behavior so that CLUSTER RESET removes the data set
of a slave node during a reset, in the moment it gets turned into a master,
so the new pattern is simply:

    FLUSHALL (that may fail for slaves)
    CLUSTER RESET