Added an option to exclude some test sets from selected mode.

Now, user can exclude some set of tests by specifying set.

Available sets:
OVS: All openvswitch network tests
BRIDGE: All bridge network tests
OPS: All VM operation tests
LSTOR: All local storage tests
CPU: All CPU tests
Merge pull request #59 from bpwook/opt_backend

Added an option to exclude some test sets from selected mode.