Import subprocess in cache devstack element

Change fd9c80208c inadvertently
removed the subprocess import from 55-cache-devstack-repos even
though it was used in a function defined within that element, unlike
in its snapshot prep script counterpart. Readd it so that this
element works again.

Change-Id: I954a1e7e99af96b08e0a59e99e513902731fd773

nodepool/elements/cache-devstack/extra-data.d/55-cache-devstack-repos

@@ -17,6 +17,7 @@
 # limitations under the License.
 
 import os
+import subprocess
 import sys
 
 # this is a bit weird; we want to get the filter to use on the