Refactor metadata CRUD functions to simplify code

[adambarreiro]

Enhancements

• Adds the POST operation for metadata, called MergeMetadata and MergeMetadataAsync as it is what it does: Updates existing keys, and adds the non-existing ones. The operation was added to all existing supported VCD entities. The input parameter is the Metadata typedValue and a map with string keys and interface{} values, in order to support the different typed values that metadata can handle in VCD.

Refactoring

• Moved deprecated functions to the bottom, now they can be easily deleted when a new major comes, as before they were mixed up with new ones.
• Ordered the functions that were a bit chaotic-ordered. Now they respect the same order always.
• Moved the getAdminUrl() to a more general place, modified its implementation to avoid coupling with Vdc and also avoid a bug when the method is called with sysadmin user.
• Deleted a lot of duplicated code in metadata tests.
• Deleted test Test_DeleteMetadataForXxxx. Now both Add/Delete are in the same test: Test_AddAndDeleteMetadataForXxx to avoid dangling metadata. Anyway, these test will be removed in next major as they test deprecated methods.
• Fixes findFirstVapp() testing function that didn't return always the defined in configuration vApp. Now it does if present.

Testing

Executed go test -v -tags "metadata" on VCD 10.3.2

[Didainius]

I hope I didn't miss some tiny value in this maze, but the tests do pass and Terraform should give another layer of check.

A few notes on capitalization in comments (I was constantly overlooking these but @lvirbalas hopefully fixed them in my head for long):

• Vdc -> VDC (Virtual Data Center)
• Org (short for Organization)

[vbauzys]

LGTM this maze :) we will see in terraform.