Added 'Tags' and supporting functions to VPC struct

[SphenicPaul]

Summary

Updated vpc.go. and vpc_test.go to include support for Tags to resolve #995 (just for VPC, not Subnet):

• Vpc struct now has additional, Tags property
• Added additional GetTagsForVpc() and GetTagsForVpcE() functions (to obtain tags for VPC)
• Added additional GetTagValueForVpc() and GetTagValueForVpcE() functions (to obtain tag value for VPC tag)
• Tests added for GetTagsForVpc() and GetTagValueForVpc().
• Test added for GetTagValueForVpcE() to assert error where no tag of specific key exists

Test Evidence

[SphenicPaul]

Please shout if you've any questions/suggestions etc. - I'm relatively new to Go but I've tried to follow conventions already present within the module.

[yorinasub17]

Thanks for your contribution! This mostly LGTM. Had a few nits, mostly around style. Once addressed, I think this is good to go!

Let me kick off a regression test build so you have one test cycle.

[SphenicPaul]

Thanks @yorinasub17 - Updated those now.

Let me know if there are any further tweaks/changes required,

[yorinasub17]

Updates LGTM! The build failed, but the test failures are known flaky tests that are unrelated to these changes, so can merge this in!