Fix an error accessing the cfg
Fix an error accessing the cfg

Since the context is passed to LogicalUnit, rather than the cfg, we can
only access the cfg as self.cfg, self.context.cfg, or context.cfg (in
the constructor). cfg is not valid anymore.

Reviewed-by: iustinp

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