Fix/Grant update failed: not allowed to create a user with grant

[alereca]

Description of your changes

Fixes #127

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.

How has this code been tested

• Manually, by creating grants for users with host=localhost and then checking if the resource reconciliation was successful.
  • Also checked in dbs whether grants were applied
• Wondering if I should update ./cluster/local/integration_tests.sh to test this scenario, since unit tests do not seem to be a good fit for this