Prevent allocations on drained nodes
Prevent allocations on drained nodes

This patch adds checks for drained nodes in the logical units that
allocate or move instances around. We also update an error message (not
style-compliant).

Reviewed-by: imsnah

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