Horizon performance tuning

Closes-Bug: #1277292

Some optimizations for Apache/WSGI settings aimed to improve
Apache and Horizon performance:
    - Apache MPM switched from 'prefork' to 'event' for RH-like
      systems
    - Apache/WSGI parameters configuration based on server's
      hardware

Change-Id: Idbd3ccaf05d9ad4a1e8206facece0c53a3fd90e7
Merge "Horizon performance tuning"