Minor improvements to Admin classes

[sl-at-ibm]

• the internal admin calls (e.g. polling calls when creating) do not seem to inherit the commandoptions (and anyway there is the matter of per-request vs overall timeouts and such)
• add a pre-polling can-poll check (like other clients), "can this token poll for DB status? (using a random guid)". Do this even before creating/dropping the db/keyspace, if waitFor is required (alignment with other clients)
• in DatabaseAdminAstra, CreateKeyspaceAsync (and probably drop ks) poll based on listKeyspaces. Arguably they should do so based on MAINTENANCE->ACTIVE state change (can borrow logic/share tool from dbsAdmin perhaps).
  • Also there's a client-side check for preexisting ks, remove it.
• DatabaseInfo.cs:35, defaults to AWS (probably should have no default). Unrelated to creating but worth noting for the get-db-info path.