Recent devstack commit [1] broke manila devstack plugin compatibility.
So, fix it by removing dependency on deleted env var in the same way
as it is done in devstack [2].
[1] I7c66e1d8d65f562596543ed8ca402dba8c8ea271
[2] I4e5c7e86aefe72fc21c77d423033e9b169318fec
Change-Id: I351f90a60e4693300cc3d3bbd1183bd8fa6acc9a
Closes-Bug: #1644194