Updates required to WP agent DA `region` variable

[ocofaigh]

• It needs to be more clear that the region is the region of the WP instance -> https://github.com/terraform-ibm-modules/terraform-ibm-scc-da/blob/de6e82ac25a113ec131964944a89ca51008ad4fc/solutions/agents/variables.tf#L40
• There should be a dropdown box with all supported regions in the ibm_catalog.json for region