Unable to sync plugin with correct metadata after syncing with incorrect

[Ekaterina Khomyakova]

Fuel 9.2 #549

Steps to reproduce:
  1. Install any plugin to fuel master node
  2. Change package_version to '11' in metadata (/var/www/nailgun/plugins/${plugin}/metadata.yaml)
  3. `fuel plugins --sync` (Plugins were successfully synchronized.)
  4. Change package_version to '1.0.0' in metadata
  5. `fuel plugins --sync`

Expected results:
  Plugins were successfully synchronized.
  Also may be there should be some errors after step 3.

Actual result:
  400 Client Error: Bad Request for url: http://10.109.0.2:8000/api/v1/plugins/sync/ (Plugin id=6 package_version=11 is not supported by Nailgun, currently supported versions 4.0., 2.0., 5.0., 3.0., 1.0.)

Workaround is to unregister plugin, change matadata, and register again, but there should be no clusters with enabled plugin.