Path component for issuer identifier should be disabled by default

[jgrandja]

Using path components in the URL of the issuer identifier enables supporting multiple issuers per host in a multi-tenant hosting configuration.

Support for path components was added in gh-1342 and is enabled by default.

However, this feature should be disabled by default, since there are a few implementation details required in order to provide a fully multi-tenant capable authorization server. Some of the primary requirements include isolation of client registration and authorization data between tenants, as well as, signing keys used per tenant.

Related gh-1342, gh-663