fix several gardenctl ssh aws issue

[neo-liang-sap]

What this PR does / why we need it:
fix several gardenctl ssh aws issue:

• if we get VPC from host, there might be several VPCs but we didn't check it, hence changing to get vpc from subnet
• there might be a list of images, use the first one
• there's some issue in golang native check network function net.DialTimeout (at least in my testing), sometimes the timeout value passed doesn't take effect, the DialTimeout function will fail immediately , hence add time.Sleep to force wait 10s to retry

Which issue(s) this PR fixes:
Fixes #466

Special notes for your reviewer:

thanks for @mandelsoft @DockToFuture catching this, @dansible FYI

Release note:

fix several gardenctl ssh aws issue:
- if we get VPC from host, there might be several VPC but we didn't check it, hence changing to get vpc from subnet
- there might be a list of images, use the first one
- there's some issue in golang native check network function `net.DialTimeout` (at least in my testing), sometimes the `timeout` value passed doesn't take effect, the DialTimeout function will fail immediately , hence add time.Sleep to force wait 10s to retry

[dansible]

I'm getting this error when testing against gctl-aws:

(1/4) Fetching data from target shoot cluster
2020/11/25 12:01:16 AWS CLI failed with
An error occurred (AuthFailure) when calling the DescribeSubnets operation: AWS was not able to validate the provided access credentials

exit status 254
exit status 1

[dansible]

NVM - i had some AWS environment variables set that caused the above issue for me.

This is working for me 👍

[neo-liang-sap]

rebase according to latest master branch, tested as follows