Use absolute path in install_pip macro
Use absolute path in install_pip macro

This lets us use the macro in the case there was a WORKDIR instruction
that changed the working directory from / earlier in the dockerfile.

Change-Id: I2369c9997ef189e7084fac7b5ace014f6f76d698
Closes-Bug: #1746682

openstack/kolla,stackforge/kolla,stackforge/kolla,stackforge/kolla,openstack/kolla,rahulunair/kolla,rahulunair/kolla