Fix 'zpool create|add' replication level check

When the pool configuration contains a hole due to a previous device
removal ignore this top level vdev. Failure to do so will result in
the current configuration being assessed to have a non-uniform
replication level and the expected warning will be disabled.

The zpool_add_010_pos test case was extended to cover this scenario.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6907
Closes #6911