Install cryptsetup on the n-cpu nodes

test_encrypted_cinder_volumes.* tempest test failes,
when cryptsetup package is not installed.

The following error can be seen in the n-cpu log:
Stderr: u'/usr/bin/nova-rootwrap: Executable not found: cryptsetup
(filter match = cryptsetup)\n'

Change-Id: I86603f1301fa946c8bb22de3e69a2ec1ab7f1ef3
diff --git a/files/debs/n-cpu b/files/debs/n-cpu
index 5d5052a..ffc947a 100644
--- a/files/debs/n-cpu
+++ b/files/debs/n-cpu
@@ -5,3 +5,4 @@
 sysfsutils
 sg3-utils
 python-guestfs # NOPRIME
+cryptsetup