DIB: relax check on pip executable
DIB: relax check on pip executable

On a devstack, pip can be installed system-wide but not coming from
system packages. So instead of checking python-pip package installation,
we only check that "which pip" succeeds

Change-Id: If8a1b7b4b7336cab0bef755b90924e03f1497b82

openstack/octavia,openstack/octavia,openstack/octavia