Add fallback to fetch limits from EC2 API

[mogren]

Description of changes:

• For unknown instance types, try calling the EC2 API
• Note: This requires the nodes to allow calls to DescribeInstanceTypes or the fallback won't work. We need to update the managed CNI policy.

I deleted m5.xlarge from the generated limits file in order to test this. The following error will be seen:

2019-12-31T02:50:53.787Z [ERROR] 	%!(EXTRA *awserr.requestError=UnauthorizedOperation: You are not authorized to perform this operation.
	status code: 403, request id: 2e2986ac-985d-4de3-8e83-4b8c25ddd63c)
2019-12-31T02:50:53.787Z [ERROR] 	Failed to get ENI limit
2019-12-31T02:50:53.787Z [ERROR] 	Initialization failure: Failed calling DescribeInstanceTypes for `m5.xlarge`: UnauthorizedOperation: You are not authorized to perform this operation.
	status code: 403, request id: 2e2986ac-985d-4de3-8e83-4b8c25ddd63c

After adding "ec2:DescribeInstanceTypes" permission it starts up correctly, even with missing instance data:

2019-12-31T02:56:56.098Z [INFO] 	Starting L-IPAMD v1.6.0-fallback-dirty  ...
2019-12-31T02:56:56.122Z [INFO] 	Testing communication with server
2019-12-31T02:56:56.123Z [INFO] 	Running with Kubernetes cluster version: v1.14+. git version: v1.14.9-eks-c0eccc. git tree state: clean. commit: c0eccca51d7500bb03b2f163dd8d534ffeb2f7a2. platform: linux/amd64
2019-12-31T02:56:56.123Z [INFO] 	Communication with server successful
2019-12-31T02:56:56.123Z [INFO] 	Starting Pod controller
2019-12-31T02:56:56.123Z [INFO] 	Waiting for controller cache sync
2019-12-31T02:56:56.124Z [DEBUG] 	Discovered region: us-west-2
2019-12-31T02:56:56.125Z [DEBUG] 	Found availability zone: us-west-2d
2019-12-31T02:56:56.125Z [DEBUG] 	Discovered the instance primary ip address: 10.10.20.28
2019-12-31T02:56:56.126Z [DEBUG] 	Found instance-id: i-0d51a749cc2eeefb0
2019-12-31T02:56:56.126Z [DEBUG] 	Found instance-type: m5.xlarge
2019-12-31T02:56:56.127Z [DEBUG] 	Found primary interface's MAC address: 0e:c8:41:0f:8d:64
2019-12-31T02:56:56.127Z [DEBUG] 	Discovered 2 interfaces.
2019-12-31T02:56:56.128Z [DEBUG] 	Found device-number: 0
2019-12-31T02:56:56.128Z [DEBUG] 	Found account ID: 973117571331
2019-12-31T02:56:56.129Z [DEBUG] 	Found eni: eni-0ebb53b6e6774b4a7
2019-12-31T02:56:56.129Z [DEBUG] 	Found ENI eni-0ebb53b6e6774b4a7 is a primary ENI
2019-12-31T02:56:56.129Z [DEBUG] 	Found security-group id: sg-08f1036b507b30a9f
2019-12-31T02:56:56.129Z [DEBUG] 	Found security-group id: sg-0aa43d9d01804ccbb
2019-12-31T02:56:56.129Z [DEBUG] 	Found subnet-id: subnet-02d0c4ea9bd25d02f
2019-12-31T02:56:56.130Z [DEBUG] 	Found vpc-ipv4-cidr-block: 10.10.0.0/16
2019-12-31T02:56:56.130Z [DEBUG] 	Found VPC CIDR: 10.10.0.0/16
2019-12-31T02:56:56.130Z [DEBUG] 	Found VPC CIDR: 100.66.0.0/16
2019-12-31T02:56:56.130Z [DEBUG] 	Start node init

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jaypipes]

Nice little patch, Claes, thanks! :)