Clarify realm id / name and expose realm id as new realm attribute

[dmeyerholt]

Coming from discussions here #216 my shot at clarifying usage of realm name and id.
Main points are:

• the requests to the API require the usage of the realm name, not the id. this is a rather cosmetic change in keycloak/realm.go
• in the realm resource, now the "real" realm id is exposed as "internal_id" attribute
• we retain the default behaviour of creating a realm with realmId == realmName so nothing breaks new/existing realms created by the provider
• This offers the possibility to import existing realms which have a realm id that differs from the realm's name. This is relevant for possible keycloak component which use the realm id for the parent_id attributes. Additional PR coming for the custom_user_federation if this PR gets through. I think, the ldap stuff is also affected by possible id and name mismatches (through the use of parent_id attributes)

[mrparkers]

This looks good so far. Here are a few changes I'd also like to see before I'm ready to merge this:

• Add a test that creates a keycloak_realm and asserts that its internal_id attribute is not the same as its name.
• Add this property to the keycloak_realm data source.
• Document this new attributes on the doc pages for the keycloak_realm resource and data source.

Let me know if you need help with any of these things.

Thanks!

[dmeyerholt]

hey @mrparkers ,
had time to finish this pr with your suggestions. Skipped documentation of data source, as that doc references the realm doc for all supported attributes.
Hope you are fine with this now.
will start new pr for improving the custom_userfederation resource to leverage this new internalId attribute.

[mrparkers]

LGTM!