Accounts/Groups Index not updated after adding new account

[dpakach]

Steps to reproduce

1. Add new user account using the CreateAccount gRPC call
2. List the available accounts using ListAccount gRPC call

Expected Result

1. The newly created account Is listed in the response

Actual Result

1. The Account is created in the data store but not returned in the response

Explaination

The reason the accounts are not returned in the response is that the indexing used for retrieving groups/accounts from the file system is not updated. So it cannot find the new accounts.

Mentioned https://github.com/owncloud/ocis-accounts/blob/master/pkg/service/v0/groups.go#L327
Tests #32

[dpakach]

Fixed by #59