CloudFormation template fails if ssh keys are disabled

[exdx]

Using the apply command without enable_ssh set resulted in an error when creating the CloudFormation template here

avalanche-ops/avalancheup-aws/src/apply/mod.rs

Line 1123 in c09c566

.unwrap();

.

I believe because creating the ec2 keys are now gated behind whether the flag is set, but the template expects them to exist regardless.

[exdx]

Need to add a condition to this block: https://github.com/ava-labs/avalanche-ops/blob/main/avalanche-ops/src/aws/cfn-templates/asg_ubuntu.yaml#L529

We can use similar logic to the NLB:

avalanche-ops/avalanche-ops/src/aws/cfn-templates/asg_ubuntu.yaml

Lines 408 to 416 in c09c566

	NlbEnabledTrue:
	Fn::Equals:
	- Ref: NlbEnabled
	- "true"
	NlbTargetGroupArnEmpty:
	Fn::Equals:
	- Ref: NlbTargetGroupArn
	- ""