Typically, the main reason a reroute allocation command with allow_primary is enabled, is to force create an empty new shard because a shard (and its replicas) were lost. This can't be done today because the shard expects to have a valid index where its allocated, we need to clear its post allocation flag to make sure it is allowed to create a fresh index.
When setting cluster.routing.allocation.disable_allocation, it causes new indices primary shards to not be allocated. By default, new indices created should allow to, at the very least, allocate primary shards so they become operations. A new setting, cluster.routing.allocation.disable_new_allocation, allows to also disable "new" allocations.
closes#2258.
