retry apt operations to avoid network issues

One of the major gate failures is do to something being wrong with
the apt mirrors. The Acquire group provides an implicit retry on
network operations which seems like it might be helpful here.

Ref: http://linux.die.net/man/5/apt.conf

Related-Bug: #1286635
(cherry picked from commit e83f7785a7609284f74667e266e38e12a29b326b)

Conflicts:
	stack.sh

Change-Id: I099f47ed86ad6a3d4296bff4cce75e7f7d946d27