Permit key imports even if signature checking is disabled (RhBug:856225)

Permit key imports even if signature checking is disabled (RhBug:856225)

- Since commit 290fcbbe6b3ca2fb1d5e4a7269a32a94f8a1563a, key imports
  on transaction sets where signature checking is disabled would fail
  due to keyring not getting loaded at all. A regression of sorts,
  in other words. As a minimal fix, temporarily enable sigcheck vsflags
  during keyring loading.