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
Merge "Set default volume backing file size to 10G"