Migrate log_{file,dir} params to neutron class

The log_{dir,file} params was configured in the neutron class::server,
these parameters was not available classes neutron::agent::*
This patch fixes this.

(This patch must be backported to havana (for #1282296))

Closes-bug: #1282296
Change-Id: I205b0ed5db08199000ec11b2212afedf67bf2ed7