Sharding incorrectly reports being added to shard

[MiaAltieri]

Steps to reproduce

juju deploy ./*charm --config role="config-server" config-server-one -n2
juju deploy ./*charm --config role="shard" shard-one -n2
juju deploy ./*charm --config role="shard" shard-two -n2
juju integrate config-server-one:config-server shard-one:sharding
juju integrate config-server-one:config-server shard-two:sharding
juju remove-relation config-server-one:config-server shard-two:sharding

Expected behavior

the last unit of shard-two to report active and running

Actual behavior

the last unit of shard-two reports it is still being added to the cluster

why this is happening

This is happening because we check if a shard has been added by relation data

proposed fix

Actually ask mongodb if the shard has been added instead of checking a boolean flag

[github-actions]

https://warthogs.atlassian.net/browse/DPE-2890