update test_volume_boot_pattern to assign one value

In commit 6809bab06e5b29efe02c11b9f4e53707c26eb1f9, the
create_snapshot function was changed to return one value.
The test_volume_boot_pattern is currently being skipped because of
a bug, so this was noticed during local testing with the test
enabled.

Related to blueprint clients-return-one-value

Change-Id: I18a68a0a4d25c8f965664d04d83dbb0842fb7037
1 file changed