Change declaration of hosts option to add functionality

[TheVastyDeep]

PR 12051 added the ability to substitute a list of URIs from an environment variable. It specifically mentions it would be useful for the hosts option in ES Input/Filter/Output plugins. It works for the input and output, but not for the filter because the filter defines the hosts option as string, not uri. But the hosts option for all three is a list of host:port combos, just filter declares it differently so it does not get the ability to substitute a list of hosts from the environment.