Example usage:
```bash
# Create a 2-node cluster
./cluster_mgr.py --action=create --replicas_per_master=1 --num_master=2
# Move (no migration) all slots to first node
./cluster_mgr.py --action=move --target_port=7001 --slot_start=8192 --slot_end=16383
# Fill data - like run memtier
# Migrate all slots to 2nd node. One could measure how long this step takes.
./cluster_mgr.py --action=migrate --target_port=7002 --slot_start=0 --slot_end=16383
```
Now this management script can:
* Create a cluster (before this PR)
* Print an existing cluster configuration
* Shutdown an existing cluster
* Move slots between cluster nodes
To support connecting to a cluster (for all new functions), I had to
change the way admin ports are defined. Instead of having the user
(optionally) specify the first port, they are hard-coded to be the
regular port + 10,000. This is done because we can't detect the admin
port based for an existing cluster (like via `CLUSTER SHARDS`).
This script allows easily setting up a local cluster.
Example invocation:
```
killall dragonfly; ./cluster_mgr.py --num_masters=3 --with_replicas
Setting up a Dragonfly cluster:
- Master nodes: 3
- Ports: 7001...7003
- Admin ports: 8001...8003
- Replicas? True
Starting nodes...
- Log file for node 7001: /tmp/dfly.cluster.node.7001.log
- Log file for node 7002: /tmp/dfly.cluster.node.7002.log
- Log file for node 7003: /tmp/dfly.cluster.node.7003.log
- Log file for node 7004: /tmp/dfly.cluster.node.7004.log
- Log file for node 7005: /tmp/dfly.cluster.node.7005.log
- Log file for node 7006: /tmp/dfly.cluster.node.7006.log
Configuring replication...
- Response for 7004: OK
- Response for 7005: OK
- Response for 7006: OK
Getting IDs...
- ID for 7001: acefdc2da5d397cfcb99239b3c29cbe6ff10d75a
- ID for 7002: a8cc67dfa42e91a94bd7c0903df35d60a39508bd
- ID for 7003: 1ad91af7bd96c89a8da877164b2ebb4cf458cab8
- ID for 7004: d209c3603343e25a18c78bd68304b6d883973bd3
- ID for 7005: bd2b25e95aaf7fdd2b955e50a00093a8272954bf
- ID for 7006: beb5cb07b75c33e3ff938d07725f2688d9bc91e0
Pushing config...
- Push into 7001: OK
- Push into 7002: OK
- Push into 7003: OK
- Push into 7004: OK
- Push into 7005: OK
- Push into 7006: OK
```
