Add possibility to select aws region to validate. usefull when using govcloud

[cpanato]

This PR adds a possibility to select the aws region when adding the aws credential.
Useful when adding the govcloud.

before was hardcoded using only us-east-1

fix: #35

[nowickjo]

really need this

[nowickjo]

still really needing this

[Dohbedoh]

Created https://issues.jenkins-ci.org/browse/JENKINS-50529

[olivereri]

Really need this too! 7 Months on.

[cpanato]

i have no influence, and I also would like this in. still waiting some feedback

[cpanato]

I will fix the conflict if the core team said this will be merged, otherwise will keep like this.

ping @andresrc

[andresrc]

Hi @cpanato , please solve the conflicts and I'll take care of the review and if everything goes well, merge and release. Sorry for the delay, my availability has been quite low.

[cpanato]

@andresrc thanks for the reply, I've fixed the conflict.

[andresrc]

Thanks for addressing the issues, I have added a couple of additional comments.

[andresrc]

Just a small additional comment. Thanks

[dakuhlman]

@cpanato @andresrc Unfortunately this still does not address the Region requirement in either of the getCredentials methods. I have created updated code for you to review and include in your PR that I believe will fix the issue with the getCredentials methods:
https://gist.github.com/dakuhlman/728812d7e426ac5f5dc3286b2b49bd63

If you would rather, I can create a separate PR to address this, but I believe it is most appropriate to be included in this PR if possible.

I have also updated the constructor and credential validation code to support awsRegion properly.

[ams-tech]

@dakuhlman Can you merge this PR and create a new one with your updates? I need this functionality.

[skhadka-securus]

Any updates ? I am still recieving error trying to deploy to gov cloud.

[cpanato]

will close this PR, i dont even remember how i did that, and the others I've opened get lost as well :(