load_gen: Supporting loading of load_gen via loader.

load_gen: Supporting loading of load_gen via loader.

Currently all tools are loaded via loader utility, except load_gen.
Load_gens were loaded directly through component_factory.
This patch adds support to load load_gens through loader utility.

The changes are as follows:
1. Configuration changes:The common.conf include configuration of
   directory. testcases.conf includes changes to load configuration,
   where tool is no more part of the load-configuration. The
   custom.conf has configuration of LOADGEN to be used - this configuration
   replaces the earlier 'tool' configuration parameter.
2. loader_utility_changes:  In loader.py, loadgen_loader is
   defined, which is used in new get_loadgen function.
3. component_factory changes: in create_loadgen, similar
   to other tools, the function just retuns the object of loadgen_class.
4. Renaming of Dummy load_gen: Loader fails to load properly a dummy
   loadgen due to name-clash with dummy in pkt_gen. To avoid this name clash
   dummy is renamed to dummyloadgen.
5. testcase changes: create_loadgen is now called with output of
   loader's get_loadgen_class.
6. Fixed Pylint Errors and extra-space at the end.
7. Included CLI options support for --loadgen and --list-loadgens.
   Thanks to Martin K.
8. Added the missing loadgen parameter in testcases.conf.
9. Fixed the missing comma error.
10. Added CI change in build-vsperf.sh
11. Fixed configuration reading in stress/stress.py

JIRA: VSPERF-533

Change-Id: I3fbb259618825a12fef55320a748a4f02509190b
Signed-off-by: Sridhar K. N. Rao <sridhar.rao@spirent.com>
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>