testcase: scalability - configurable installation of flows to the vswitch

testcase: scalability - configurable installation of flows to the vswitch

Stream specific flows can be pre-installed into the vswitch
based on the value of testcase specific configuration option
"Pre-installed Flows". In case, it is set to 'Yes', then
specific flow for each stream will be inserted into the switch.
Otherwise only generic flows will be installed. Default value
of "Pre-installed Flows" is set to 'No'. Its value can be
overridden by CLI parameter pre-installed_flows.
This configuration parameter is an enhancement of "MultiSream"
feature and it is ignored if "MultiStream" is disabled.
Python module 'netaddr' is required by this implementation
and it has been added to requirements.txt file.

Change-Id: I8a17577a702bf2be2753134eb203b936a87fc2e5
JIRA: VSPERF-83
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Brian Castelli <brian.castelli@spirent.com>