Consider the host_ip value when handling port collisions

In response to issue #4608, this fix allows vagrant to handle forwarding port collisions by considering the :host_ip flag as well. This allows vagrant to bind to multiple interfaces on host machine (or loopbacks) if there are any.
For example:
guest1 forwards port 80 to 127.1.1.253:80 on host
guest2 forwards port 80 to 127.1.1.254:80 on host

Previously vagrant was not considering that host can have multiple interfaces to forwards ports to.