Use IMDSv2 to get instance metadata -ELB

Issue #, if available:
This is similar to other PR on elbv2  aws-samples#102 
Current version of script breaks when IMDSv2 is enforced on EC2 instances(
"InstanceMetadataOptions": {
"HttpTokens": "required",
} )

Description of changes:
To improve security, AWS recommend to use IMDSv2, the session-oriented communication to get instance metadata.https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html. This change is backward compatible with IMDSv1.
Also updated the request to get the region using placement/region this feature was released by AWS on 2020-08-24 - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instancedata-data-categories.html

Author: rajeshfintech

load-balancing/elb/common_functions.sh

@@ -97,9 +97,8 @@ exec_with_fulljitter_retry() {
 #   Writes to STDOUT the AWS region as known by the local instance.
 get_instance_region() {
     if [ -z "$AWS_REGION" ]; then
-        AWS_REGION=$(curl -s http://169.254.169.254/latest/dynamic/instance-identity/document \
-            | grep -i region \
-            | awk -F\" '{print $4}')
+        TOKEN=$(curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600")
+        AWS_REGION=$(curl -H "X-aws-ec2-metadata-token: $TOKEN" -s http://169.254.169.254/latest/meta-data/placement/region)
     fi
 
     echo $AWS_REGION
@@ -706,6 +705,7 @@ error_exit() {
 #   Writes to STDOUT the EC2 instance ID for the local instance. Returns non-zero if the local
 #   instance metadata URL is inaccessible.
 get_instance_id() {
-    curl -s http://169.254.169.254/latest/meta-data/instance-id
+    TOKEN=$(curl -X PUT "http://169.254.169.254/latest/api/token" -H "X-aws-ec2-metadata-token-ttl-seconds: 21600")
+    curl -H "X-aws-ec2-metadata-token: $TOKEN" -s http://169.254.169.254/latest/meta-data/instance-id
     return $?
 }