Fix bug where updates to config would fail is password isn't provided

[jasonodonnell]

If a user tries to update the config and doesn't provide the binddn or bindpass, it will fail with an error: cannot derive UserBindDN. This PR adds a read operation to update to first check storage for a config and pass it into the config builder. I've added a test to cover this, which fails without the patch:

$ go test -v -run TestBackend
=== RUN   TestBackend
=== RUN   TestBackend/write_config
=== RUN   TestBackend/read_config
=== RUN   TestBackend/delete_config
=== RUN   TestBackend/update_config
    backend_test.go:119: cannot derive UserBindDN
=== RUN   TestBackend/plant_config
=== RUN   TestBackend/write_role
=== RUN   TestBackend/read_role
=== RUN   TestBackend/list_roles
=== RUN   TestBackend/delete_role
=== RUN   TestBackend/plant_role
=== RUN   TestBackend/read_cred
=== RUN   TestBackend/rotate_role_creds
=== RUN   TestBackend/rollback_role_creds
=== RUN   TestBackend/discard_WAL
=== RUN   TestBackend/rotate_root_creds
=== RUN   TestBackend/rotate_root_creds_with_write
--- FAIL: TestBackend (0.01s)
    --- PASS: TestBackend/write_config (0.00s)
    --- PASS: TestBackend/read_config (0.00s)
    --- PASS: TestBackend/delete_config (0.00s)
    --- FAIL: TestBackend/update_config (0.00s)
    --- PASS: TestBackend/plant_config (0.00s)
    --- PASS: TestBackend/write_role (0.00s)
    --- PASS: TestBackend/read_role (0.00s)
    --- PASS: TestBackend/list_roles (0.00s)
    --- PASS: TestBackend/delete_role (0.00s)
    --- PASS: TestBackend/plant_role (0.00s)
    --- PASS: TestBackend/read_cred (0.00s)
    --- PASS: TestBackend/rotate_role_creds (0.00s)
    --- PASS: TestBackend/rollback_role_creds (0.00s)
    --- PASS: TestBackend/discard_WAL (0.00s)
    --- PASS: TestBackend/rotate_root_creds (0.00s)
    --- PASS: TestBackend/rotate_root_creds_with_write (0.00s)
FAIL
exit status 1
FAIL	github.com/hashicorp/vault-plugin-secrets-ad/plugin	1.123s

[austingebauer]

👍