Implementation description management (RDSv3)

[bakhterets]

What does this PR do?

Adds support for updating the description (alias) of an RDS v3 instance.

Changes:

• Added UpgradeDescription function and UpgradeDescriptionOpts struct to openstack/rds/v3/instances.
• Added unit tests for the new endpoint using HTTP mocking (requests_test.go).
• Added an acceptance test step to TestRdsLifecycle to verify the functionality against the real API.

Closes issues/929

[anton-sidelnikov]

add check to verify alias/description like:

	getRdsDesc, err := instances.List(client, instances.ListOpts{
		Id: rds.Id,
	})
	th.AssertNoErr(t, err)
	th.AssertEquals(t, newRds.Instances[0].Alias, newAlias)