While Building 50 Servers with 10 AG Groups in each Server we had to uninstall SQL and re-install again to address one of the server configuration issue. While re-installing we had challenges creating the availability group.
Final solution is to delete the AGNames from Failover Cluster Manager (Resources) and delete registry entry @ HKEY_LOCAL_MACHINE\Cluster\HadrAgNameToldMap.
Interestingly, when we create first Availability group, all 10 Registry entries were restored @ HKEY_LOCAL_MACHINE\Cluster\HadrAgNameToldMap. We had to delete appropriate entry from Registry and create availability group, repeat this process for all 10 AGs being created.
Referring to https://connectbeta.microsoft.com/SQLServer/feedback/details/2451275/uninstall-does-not-remove-ag-registry-hadragnametoldmap-and-cluster-entries, workaround/recommendation is to delete the AGs before uninstalling the SQL Server.