controller: set external name to its latest value no matter what

[muvaf]

Description of your changes

It's up to discussion whether there should be a re-creation behavior but until that lands, we'll conform to how native controllers behave and override existing external name if we have another, like the case where VPC is deleted in the console and we re-create it since its query returns 404.

Fixes https://github.com/crossplane/terrajet/issues/122

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

• Created a VPC and it got into ready state.
• Deleted the VPC in console.
• Validated that a new VPC is created and external name annotation contained that new VPC ID, not the old one.
• Deleted and both CR and VPC were gone.