node evac: don't call IAllocator if no instances
node evac: don't call IAllocator if no instances

Currently we generate an empty list only for the '-n node' invocation,
but for iallocator we still call the iallocator (which needs an RPC
call, etc.). By moving the computation of instances outside of the if
block, we can return early from the LU.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

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