nodepool: provide openstack env variables to system user

Set some OS_* env variables in nodepool .profile, let us easily interact
with the OpenStack API since the CLI utilities recognizes the variables.

Using shell_exports() from puppet wmflib module which has been used in
our apache module already.

Bug: T103673
Change-Id: Ia912f444af2dd2a0fab3b257e01b317f880e69f9