Cleanup neutron provider router example

* Don't define keystone admin variables, they are not needed and would
  override the other variabels
* Define variables in 'adminrc' instead of 'novarc' (nova is not used in
  the section)
* Don't automate the sourcing of 'adminrc' in bashrc since the admin
  might not want to always be logged in the admin tenant
* Use the 'admin' tenant as provider tenant, since this is the one that
  was created earlier in the guide. Explain why this tenant is used.
* Try to respect conventions
* Improve commands used to create network objects:
  - set dns servers for tenant_A subnet
  - explicitly define the provider network configuration

Change-Id: Ice2db30360a3fbecd64237bc2a3c97a9d0e4b0d9
Closes-Bug: #1266643
Merge "Cleanup neutron provider router example"