Fix another regression: a crash introduced in #79 (feb892f)

That commit introduced a NullPointerException crash:

After the first pass through the `computeResourcesForSlot()` main loop, the
`executorCpuResources` and `executorMemResources` variables are not populated,
and thus incorreclty null, because `subtractedExecutorResources ` was set to
true on the 1st pass.

The fix is to unconditionally set them to the requested resource amounts
unconditionally, and only subtract the resources out of the offer on the
1st pass.

Also an unrelated change:  clarify that the reservations that are being skipped
in a couple functions are *dynamic* reservations.

src/main/storm/mesos/MesosNimbus.java

@@ -306,7 +306,7 @@ protected OfferResources getResources(Offer offer, double executorCpu, double ex
 
     for (Resource r : offer.getResourcesList()) {
       if (r.hasReservation()) {
-        // skip resources with reservations
+        // skip resources with dynamic reservations
         continue;
       }
       if (r.getType() == Type.SCALAR) {
@@ -495,7 +495,7 @@ protected List<Resource> getResourcesScalar(final List<Resource> offerResources,
     double valueNeeded = value;
     for (Resource r : offerResources) {
       if (r.hasReservation()) {
-        // skip resources with reservations
+        // skip resources with dynamic reservations
         continue;
       }
       if (r.getType() == Type.SCALAR &&
@@ -739,13 +739,11 @@ protected void computeResourcesForSlot(final RotatingMap<OfferID, Offer> offers,
           Collections.sort(offerResources, new ResourceRoleComparator());
         }
 
-        List<Resource> executorCpuResources = null;
-        List<Resource> executorMemResources = null;
+        List<Resource> executorCpuResources = getResourcesScalar(offerResources, executorCpu, "cpus");
+        List<Resource> executorMemResources = getResourcesScalar(offerResources, executorMem, "mem");
         List<Resource> executorPortsResources = null;
         if (!subtractedExecutorResources) {
-          executorCpuResources = getResourcesScalar(offerResources, executorCpu, "cpus");
           offerResources = subtractResourcesScalar(offerResources, executorCpu, "cpus");
-          executorMemResources = getResourcesScalar(offerResources, executorMem, "mem");
           offerResources = subtractResourcesScalar(offerResources, executorMem, "mem");
           subtractedExecutorResources = true;
         }