Fix various things when n-cell is enabled

1) Use the newer cell_type configuration option instead of overriding
the compute_api_class.

2) A nova-cells service is started for both the API cell (region) and
the compute cell (child).  The screen names were both 'n-cell' which
caused both services to log to the same log file.  The screen names have
been changed to n-cell-region and n-cell-child.

3) Security groups are not supported with cells.  Multiple exercises are
patched to use the 'default' security group instead of creating a new
one.  Additionally, if one uses the 'default' security group, do not try
to delete it, as one can never delete the default.

4) Disable the floating_ips and aggregates exercises when n-cell is
enabled, as they are not supported by cells.

5) Related to #4, disable the floating_ips tests within euca.sh

6) Update the README.md.  No services need to be disabled, and one only
needs to enable the n-cell service.

Change-Id: I9782d1e3cda3c9dd3daefa15c043f5b06473cb87
diff --git a/exercises/boot_from_volume.sh b/exercises/boot_from_volume.sh
index 1814732..a3a14eb 100755
--- a/exercises/boot_from_volume.sh
+++ b/exercises/boot_from_volume.sh
@@ -80,12 +80,18 @@
 # List security groups
 nova secgroup-list
 
-# Create a secgroup
-if ! nova secgroup-list | grep -q $SECGROUP; then
-    nova secgroup-create $SECGROUP "$SECGROUP description"
-    if ! timeout $ASSOCIATE_TIMEOUT sh -c "while ! nova secgroup-list | grep -q $SECGROUP; do sleep 1; done"; then
-        echo "Security group not created"
-        exit 1
+if is_service_enabled n-cell; then
+    # Cells does not support security groups, so force the use of "default"
+    SECGROUP="default"
+    echo "Using the default security group because of Cells."
+else
+    # Create a secgroup
+    if ! nova secgroup-list | grep -q $SECGROUP; then
+        nova secgroup-create $SECGROUP "$SECGROUP description"
+        if ! timeout $ASSOCIATE_TIMEOUT sh -c "while ! nova secgroup-list | grep -q $SECGROUP; do sleep 1; done"; then
+            echo "Security group not created"
+            exit 1
+        fi
     fi
 fi
 
@@ -200,8 +206,12 @@
 end_time=$(date +%s)
 echo "Completed cinder delete in $((end_time - start_time)) seconds"
 
-# Delete secgroup
-nova secgroup-delete $SECGROUP || die $LINENO "Failure deleting security group $SECGROUP"
+if [[ $SECGROUP = "default" ]] ; then
+    echo "Skipping deleting default security group"
+else
+    # Delete secgroup
+    nova secgroup-delete $SECGROUP || die $LINENO "Failure deleting security group $SECGROUP"
+fi
 
 set +o xtrace
 echo "*********************************************************************"