backend: Add optional exclusion list to _CleanDirectory
backend: Add optional exclusion list to _CleanDirectory

The code cleaning the queue will make use of it.

Reviewed-by: iustinp


leshchevds/ganeti,mbakke/ganeti,onponomarev/ganeti,dimara/ganeti,dimara/ganeti,apyrgio/snf-ganeti,mbakke/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,apyrgio/ganeti,andir/ganeti,yiannist/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,leshchevds/ganeti,grnet/snf-ganeti,ganeti/ganeti,yiannist/ganeti,andir/ganeti,yiannist/ganeti,bitemyapp/ganeti