playbook(memcache): support instances and ${instances_sequence}

[EricWai]

fix #240

1. hosts.yaml support the instances.
2. add the hosts_instances.yaml as a template.
3. Fixed spelling errors in variable names: "expoter" -> "exporter".
4. add "if [ "${EXPORTER_PORT}" ];then".

Now, when deploying the memcached cluster, hosts.yaml can be written as follows：

global:
  user: curve
  ssh_port: 22
  private_key_file: /home/curve/.ssh/id_rsa
  
hosts:
  - host: server-host1
    hostname: 10.0.1.1
    labels:
      - memcached
    envs:
      - SUDO_ALIAS=sudo
      - ENGINE=docker
      - IMAGE=memcached:1.6.17
      - EXPORTER_IMAGE=quay.io/prometheus/memcached-exporter:v0.13.0
      - LISTEN=10.0.1.1
      - PORT=112${instances_sequence}
      - EXPORTER_PORT=91${instances_sequence}
      - USER=root
      - MEMORY_LIMIT=32768 # item memory in megabytes
      - MAX_ITEM_SIZE=8m # adjusts max item size (default: 1m, min: 1k, max: 1024m)
      - EXT_PATH=/mnt/memcachefile/cachefile:10${instances_sequence}G
      - EXT_WBUF_SIZE=8 # size in megabytes of page write buffers.
      - EXT_ITEM_AGE=1 # store items idle at least this long (seconds, default: no age limit)
      - VERBOSE="v"
    instances: 3

  - host: server-host2
    hostname: 10.0.1.2
    labels:
      - memcached
    envs:
      - SUDO_ALIAS=sudo
      - ENGINE=docker
      - IMAGE=memcached:1.6.17
      - EXPORTER_IMAGE=quay.io/prometheus/memcached-exporter:v0.13.0
      - LISTEN=10.0.1.2
      - PORT=11211
      - EXPORTER_PORT=9151
      - USER=root
      - MEMORY_LIMIT=32768 # item memory in megabytes
      - MAX_ITEM_SIZE=8m # adjusts max item size (default: 1m, min: 1k, max: 1024m)
      - EXT_PATH=/mnt/memcachefile/cachefile:1024G
      - EXT_WBUF_SIZE=8 # size in megabytes of page write buffers.
      - EXT_ITEM_AGE=1 # store items idle at least this long (seconds, default: no age limit)
      - VERBOSE="v"

[caoxianfei1]

@EricWai Compile failed and check it.