Set default volume backing file size to 10G

This commit changes the default volume backing file size from 5G to
10G. This is already done in devstack-gate because on tempest runs
we would frequently go above 5G. This will mirror the change for all
devstack runs not just in the gate.

Change-Id: I7023237653a28a4bb9413540ad9ecf3dfa588b0a
diff --git a/stackrc b/stackrc
index ef39710..23b5bb8 100644
--- a/stackrc
+++ b/stackrc
@@ -233,8 +233,8 @@
 esac
 
 
-# 5Gb default volume backing file size
-VOLUME_BACKING_FILE_SIZE=${VOLUME_BACKING_FILE_SIZE:-5130M}
+# 10Gb default volume backing file size
+VOLUME_BACKING_FILE_SIZE=${VOLUME_BACKING_FILE_SIZE:-10250M}
 
 # Name of the LVM volume group to use/create for iscsi volumes
 VOLUME_GROUP=${VOLUME_GROUP:-stack-volumes}