Instance: add a new all_nodes property
Instance: add a new all_nodes property

Since we often need the list of all nodes of an instance, we add a new
"all_nodes" property that returns all nodes of the instance, and we
switch secondary_nodes to a simpler implementation based on this new
function.

Reviewed-by: ultrotter

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