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
Merge "Allow overriding USE_GET_PIP via env vars"