Improve abstraction of hosts

A variable is basically a host or a port, and a host can be either an IP
address, a hostname, another variable, and so on.

Use our abstraction of host_list_item outside of lists when a single
host is expected, as a variable value, or in a rule.  This allows us to
have a better abstraction.

lib/puffy/parser.y

@@ -13,10 +13,8 @@ rule
                      | variable_value_list variable_value     { result = val[0] + [val[1]] }
                      | variable_value                         { result = [val[0]] }
 
-  variable_value: ADDRESS  { result = val[0][:value] }
-                | STRING   { result = val[0][:value] }
-                | VARIABLE { result = @variables.fetch(val[0][:value]) }
-                | port     { result = val[0] }
+  variable_value: host_list_item { result = val[0] }
+                | port           { result = val[0] }
 
   service: SERVICE service_name block { @services[val[1]] = val[2] }
 
@@ -122,10 +120,9 @@ rule
                 |       { result = {} }
 
   hosts_host: ANY hosts_port               { result = [{ host: nil, port: val[1] }] }
-            | host hosts_port              { result = [{ host: val[0], port: val[1] }] }
             | hosts_port                   { result = [{ host: nil, port: val[0] }] }
+            | host_list_item hosts_port    { result = [{ host: val[0], port: val[1] }] }
             | '{' host_list '}' hosts_port { result = [{ host: val[1], port: val[3] }] }
-            | VARIABLE hosts_port          { result = [{ host: @variables.fetch(val[0][:value]), port: val[1] }] }
             | SRV '(' STRING ')'           { result = Resolver.instance.resolv_srv(val[2][:value]) }
             | APT_MIRROR '(' STRING ')'    { result = Resolver.instance.resolv_apt_mirror(val[2][:value]) }