fix warning in install_get_pip
fix warning in install_get_pip

we were unconditionally adding -z to the curl command even if the file
doesn't exist that we are referencing. That produces a scary warning
for users. Lets not do that.

Change-Id: Id2860c1c702510b0f8fd496abce579d0fa3ff867

avvocatodemarchis/devstack,Mahito/devstack,bswartz/devstack,mssumanth/devstack,richliu/devstack,dtroyer/devstack,dirkmueller/devstack,avvocatodemarchis/devstack,thomasem/devstack,dtroyer/devstack,openstack-dev/devstack,prithivm/branch01,noironetworks/devstack,dirkmueller/devstack,samgoon/devstack,openstack-dev/devstack,mssumanth/devstack,pombredanne/devstack,group-policy/devstack,bigswitch/devstack,nawawi/openstack,noironetworks/devstack,varunarya10/devstack,Mahito/devstack,samgoon/devstack,ozamiatin/devstack,pombredanne/devstack,bswartz/devstack,prithivm/branch01,nawawi/openstack,richliu/devstack,varunarya10/devstack,ozamiatin/devstack,bigswitch/devstack,thomasem/devstack,group-policy/devstack