SiteSync: Providers ignore that site is disabled

[kalisp]

Brief description

GDrive site was checking presence of credentials even if it was disabled on a project.

Description

This caused uncaught exceptions which prevented of synching of totally unrelated sites, which were configured correctly.
Same behavior noticed in Dropbox, SFTP checked also (without this issue).

Testing notes:

1. Add Gdrive site in System Settings
2. disable it on project settings
3. restart Tray (without this fix, it would complain in the log that Gdrive credentials are missing)

[ynbot]

Task linked: OP-3072 GDrive provider ignore that site is disabled

[m-u-r-p-h-y]

Previously asking for gdrive credentials even though the site was not active in the project

  - { SyncServer }: [  Sync tasks count 0  ]
>>> [  Sync Server: No credentials for gdrive provider for 'gdrive' on path ''!  ]
  - { SyncServer }: [  Check representations for : murphy_test_hierarchy  ]

Not asking for gdrive credentails anymore

  - { SyncServer }: [  Sync Server: Site gdrive not enabled for murphy_test_hierarchy.  ]
  - { SyncServer }: [  Check representations for : murphy_test_hierarchy  ]