RAPI: fix a pylint warning
RAPI: fix a pylint warning

Child classes of _R_TAGS must define TAG_LEVEL, but for good style let's
define it also here to at least ensure we don't get a 'Unknown
attribute' exception.

Of course, this also silences a pylint warning.

Reviewed-by: amishchenko

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