Adds support for LVM ephemeral storage in Nova

DevStack currently lacks support for LVM ephemeral storage in Nova.
This support is important for testing of Nova's LVM backend. The
proposed change adds a default volume group, to be shared by Cinder
and Nova. It also adds a configuration option NOVA_BACKEND, which
must be LVM if it is set, that determines whether Nova should be
configured to use LVM ephemeral storage.

Change-Id: I4eb9afff3536fbcd563939f2d325efbb845081bb
diff --git a/stack.sh b/stack.sh
index 585d1ce..45113b5 100755
--- a/stack.sh
+++ b/stack.sh
@@ -523,6 +523,7 @@
 source $TOP_DIR/lib/infra
 source $TOP_DIR/lib/oslo
 source $TOP_DIR/lib/stackforge
+source $TOP_DIR/lib/lvm
 source $TOP_DIR/lib/horizon
 source $TOP_DIR/lib/keystone
 source $TOP_DIR/lib/glance
@@ -939,6 +940,10 @@
 # A better kind of sysstat, with the top process per time slice
 start_dstat
 
+# Initialize default LVM volume group
+# -----------------------------------
+init_lvm_volume_group $DEFAULT_VOLUME_GROUP_NAME $VOLUME_BACKING_FILE_SIZE
+
 # Start Services
 # ==============