Add Network Context Class

Context for networking resources.

Note:
  using this context in security_groups related scenarios
  will be added in another patch.

Main changes:
  * the network context class
  * rally.benchmark.utils.RAMInt class that represents integer value in RAM,
    so we can make process safe sync of cluster-unique stuff (like CIDRs)
  * rally.benchmark.wrappers.network module, that unifies network service
    implementation, for usage both in context and scenarios
  * optimization in NovaScenario._boot_server

Co-Authored-By: Lingxian Kong <anlin.kong@gmail.com>
Co-Authored-By: Timur Nurlygayanov <tnurlygayanov@mirantis.com>
Co-Authored-By: Sergey Skripnick <sskripnick@mirantis.com>
Co-Authored-By: Vadim Rovachev <vrovachev@mirantis.com>
Co-Authored-By: Aleksandr Maretskiy <amaretskiy@mirantis.com>

Implements: blueprint benchmark-context-network
Change-Id: Ia2c461ba3421dc9650c4bb7949c115fee08c6b7b
Merge "Add Network Context Class"