Allow explicit interface specification for multi-homed hosts

Allow the user of the cookbook to specify which interface IP to use for
creating the Zabbix host entry's "interfaces" list. I.e., the interface
that the host will be polled on once it is registred.
Fix typo in variable assignment for ip_address
Update interface selection variable name to not conflict with other recipes
Merge branch 'master' into specify-interface-ip

Conflicts:
	recipes/agent_registration.rb
Fix Rubocop errors
Merge pull request #144 from ftclausen/specify-interface-ip

Allow specific interface to be used for agent polling.