Support for composable host metadata

[mikemol]

It would be quite handy for zabbix: zabbix-agent: hostmetadata to support composability.

For example:

zabbix:
  zabbix-agent:
    hostmetadata:
      bind-monitor:
      squid-proxy:
      mx-in:
      {{ grains['osfinger'] }}:

(I'm using dicts to support pillar merging; lists can't be merged)

This might result in a configuration line of HostMetadata=bind-monitor squid-proxy mx-in CentOS Linux 7, as an example, and would permit easier configuration of Zabbix monitoring of autodiscovered hosts.