Remove extension checks for quantum_network.

 * The extension checks were failing if /etc/quantum/quantum.conf
   did not yet exist, which could easily be the case if quantum
   installation was attempted during the same run as provisioning.
   Since the checks are pointless anyway - the provider and l3
   extensions are essential parts of the quantum api - this change
   removes them.

Change-Id: I872d56b29ee4de8fefcd1e8ee0c0666cacccb533