Fixes according to issues: #19, #20, #21 and #22.

* Ensure resources have consistent exists methods.
* Corrected a coupld of arguments in example doc strings.

Signed-off-by: Stuart Paterson <spaterson@chef.io>

libraries/google_compute_firewall.rb

@@ -51,6 +51,10 @@ def ports_protocol_allowed(port_list, protocol = 'tcp', index = 0)
       end
     end
 
+    def exists?
+      !@firewall.nil?
+    end
+
     def to_s
       "Firewall Rule #{@display_name}"
     end

libraries/google_compute_image.rb

@@ -36,6 +36,10 @@ def initialize(opts = {})
       end
     end
 
+    def exists?
+      !@image.nil?
+    end
+
     def to_s
       "Image #{@display_name}"
     end

libraries/google_compute_instance_group.rb

@@ -8,7 +8,7 @@ class GoogleComputeInstanceGroup < GcpResourceBase
     desc 'Verifies settings for a compute instance group'
 
     example "
-      describe google_compute_instance_group(project: 'chef-inspec-gcp', location: 'us-west2', name: 'gcp-inspec-test') do
+      describe google_compute_instance_group(project: 'chef-inspec-gcp', zone: 'europe-west2-a', name: 'gcp-inspec-test') do
         it { should exist }
         its('name') { should eq 'inspec-test' }
         its('status') { should eq 'in_use' }
@@ -38,6 +38,10 @@ def get_named_ports(index = 0, key = :name)
       named_ports[index].item[key]
     end
 
+    def exists?
+      !@instance_group.nil?
+    end
+
     def to_s
       "Instance Group #{@display_name}"
     end

libraries/google_container_node_pool.rb

@@ -9,7 +9,7 @@ class GoogleContainerNodePool < GcpResourceBase
     desc 'Verifies settings for a container nodepool'
 
     example "
-      describe gcp_container_nodepool(project: 'chef-inspec-gcp', zone: 'gcp-inspec-test', name: 'cluster-name') do
+      describe google_container_node_pool(project: 'chef-inspec-gcp', zone: 'gcp-inspec-test', cluster_name: 'cluster-name', nodepool_name: 'nodepool_name') do
         it { should exist }
         its('name') { should eq 'inspec-test' }
         its('status') { should eq 'in_use' }

libraries/google_service_account.rb

@@ -32,7 +32,7 @@ def initialize(opts = {})
     end
 
     def exists?
-      !@iam_role.nil?
+      !@service_account.nil?
     end
 
     def to_s