LogicalUnit._LockInstancesNodes helper function
LogicalUnit._LockInstancesNodes helper function

This function is used to lock instances' primary and secondary nodes
after locking instances themselves.

Reviewed-by: iustinp


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