Gardenctl fails for AWS - hardcoded instance type

[danielfoehrKn]

Describe the bug
Currently for AWS the instance-type t2 is hardcoded when creating the bastion instance.

Depending on the AWS region, the T2 instance type is unavailable and leads the following error.
It was not immediately clear to me that this is the raw error from the awscli:

(3/4) Creating bastion host

An error occurred (Unsupported) when calling the RunInstances operation: The requested configuration is currently not supported. Please check the documentation for supported configurations.

To Reproduce
Steps to reproduce the behavior:
Execute gardenctl ssh for a node in the AWS region eu-north-1

Expected behavior

Gardenctl should check that the T2 instance type is available in the given region of the node.
If not it should either

• return an error stating the available instance types & lets the user pick one.
• pick an available instance type

Screenshots
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Gardenctl Version (please complete the following information):

• Version/Commit [e.g. b21b4bd]

Additional context
Add any other context about the problem here.

[neo-liang-sap]

please assign this issue to me, thanks