Use right PKI defaults

The common cookbook has been updated to make PKI the default token
provider in keystone. This patch updates the specs to match that.

The default PKI key size is double what the default value in the
cookbook is. This patch updates it.

Change-Id: Ic17c572f7aa67f88b8f6f2ed31968d043f1abb73
Partial-Bug: #1312927