Rename apts to debs

In change I0416180db5b6add996ce5b48c6966c1b68adbcb0, devstack renames
apts to debs.  This updates grenade to look for packages in the correct
'files/debs' directory.

Change-Id: I915f0e59c13d8cf5fb3549c6202f8aa2f8be0856
Merge "Rename apts to debs"