Allow overriding USE_GET_PIP via env vars

devstack-gate wants to pre-cache and then use get-pip, but we can't
throw the flag currently. Make the flag default settable via env vars.

Change-Id: I661b52670b6ce494666cbdd611e4eee6b96c8321
Partial-Bug: #1254275
Move neutron cache dir into a function

Taking the model of Nova, this patch aims to move the cache directory
management into a function with the goal to reuse it somewhere else like
Grenade.

Change-Id: I93df52f69ef339e6528b88d88d4ea70e0b725893
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Allow overriding USE_GET_PIP via env vars"
Merge "Move neutron cache dir into a function"