OSFromDisk: use script names from constants
OSFromDisk: use script names from constants

Reviewed-by: iustinp


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