fix(multi-tenancy): upsert groot for namespace after drop data

[8i5dev]

Description: After executing drop data in a namespace, the namespace cannot be logged in anymore

1. drop data of a namespace

curl --location 'http://localhost:8080/alter' \
--header 'X-Dgraph-AccessToken: accesstoken' \
--header 'Content-Type: text/plain' \
--data '{"drop_op": "DATA"}
'

2. login the namespace again

curl --location 'http://127.0.0.1:8080/login' \
--header 'Content-Type: application/json' \
--data '{
    "userid": "groot",
    "password": "password",
    "namespace": 1
}'

3. get error message Authentication from address10.42.0.1:55404 failed: unable to authenticate: invalidi credentials

function InitializeAcl only upsert groot for namespace 0

[CLAassistant]

All committers have signed the CLA.

[mangalaman93]

Thank you for the PR. Is it possible for you to rebase the PR while I review it?

[mangalaman93]

Thank you for raising this issue, we have fixed it in #9159 with some tests.