Rename "virtual organization" to "virtual issuer"

[jjg-123]

OA4MP allows for creating virtual issuer, i.e., separate issuers with their own keys, iss claim and discovery point. When first created several years back, these were name "virtual organizations" internally. The problem with this is that virtual orgs do exist elsewhere and in particular, since CILogon works closely with COmanage, there is often quite some confusion that OA4MP somehow extends COmanage. It is much cleaner to rename everything, except the internal SQL column names for the stores, since such a change would play Hob with every existing installation.

The program therefore is to change the names in OA4MP, which includes renaming the classes and changing the CLI's text messages.