Don't have synapse require the aws variables - credentials should be

able to be nil to allow using iam instance profile. Updates tests to
reflect

lib/synapse/service_watcher/ec2tag.rb

@@ -45,11 +45,14 @@ def validate_discovery_opts
         raise ArgumentError, "Invalid server_port_override value"
       end
 
-      # Required, but can use well-known environment variables.
-      %w[aws_access_key_id aws_secret_access_key aws_region].each do |attr|
-        unless (@discovery[attr] || ENV[attr.upcase])
-          raise ArgumentError, "Missing #{attr} option or #{attr.upcase} environment variable"
-        end
+      # aws region is optional in the SDK, aws will use a default value if not provided
+      unless @discovery['aws_region'] || ENV['AWS_REGION']
+        log.info "aws region is missing, will use default"
+      end
+      # access key id & secret are optional, might be using IAM instance profile for credentials
+      unless ((@discovery['aws_access_key_id'] || ENV['aws_access_key_id']) \
+              && (@discovery['aws_secret_access_key'] || ENV['aws_secret_access_key'] ))
+        log.info "aws access key id & secret not set in config or env variables for service #{name}, will attempt to use IAM instance profile"
       end
     end
 

spec/lib/synapse/service_watcher_ec2tags_spec.rb

@@ -87,20 +87,20 @@ def munge_haproxy_arg(name, new_value)
     end
 
     context 'when missing arguments' do
-      it 'complains if aws_region is missing' do
+      it 'does not break if aws_region is missing' do
         expect {
-          Synapse::ServiceWatcher::Ec2tagWatcher.new(remove_discovery_arg('aws_region'), mock_synapse)
-        }.to raise_error(ArgumentError, /Missing aws_region/)
+          Synapse::ServiceWatcher::EC2Watcher.new(remove_discovery_arg('aws_region'), mock_synapse)
+        }.not_to raise_error
       end
-      it 'complains if aws_access_key_id is missing' do
+      it 'does not break if aws_access_key_id is missing' do
         expect {
-          Synapse::ServiceWatcher::Ec2tagWatcher.new(remove_discovery_arg('aws_access_key_id'), mock_synapse)
-        }.to raise_error(ArgumentError, /Missing aws_access_key_id/)
+          Synapse::ServiceWatcher::EC2Watcher.new(remove_discovery_arg('aws_access_key_id'), mock_synapse)
+        }.not_to raise_error
       end
-      it 'complains if aws_secret_access_key is missing' do
+      it 'does not break if aws_secret_access_key is missing' do
         expect {
-          Synapse::ServiceWatcher::Ec2tagWatcher.new(remove_discovery_arg('aws_secret_access_key'), mock_synapse)
-        }.to raise_error(ArgumentError, /Missing aws_secret_access_key/)
+          Synapse::ServiceWatcher::EC2Watcher.new(remove_discovery_arg('aws_secret_access_key'), mock_synapse)
+        }.not_to raise_error
       end
       it 'complains if server_port_override is missing' do
         expect {