Switch RHOSTS to TARGETS and add validation

Hi @morisson,

This pull request tries to modify https://github.com/rapid7/metasploit-framework/pull/2791 by:

* Switching from RHOSTS to TARGETS, since RHOSTS hasn't the expected behaviour exactly, I think keep using RHOSTS could lead to confusions. So I've switched the datastore option name to TARGETS. Anyway, my opinion isn't hard here. So if you prefer RHOSTS, and other developers don't complain I won't fight for TARGETS :) But at the moment looks like the best option for me.

* Adds some validation to TARGETS when resolution is remote, at least to verify a comma separated list of valid addresses and/or hostnames is provided.

Please, review, discuss, ask, and check anything, once comfortable land this pull request and the original one https://github.com/rapid7/metasploit-framework/pull/2791 will be automatically updated. Thanks!