virttest.staging.service: Introduce a class named ServiceManagerFactory.

Introduce a class named ServiceManagerFactory to create kinds of service manager.
* Class ServiceManagerFactory provides two staticmethod to create generate service manager
and to create specific service manager.
* Internal class named FactoryHelper to provide some help method for ServiceManagerFactory.

Relationship of the classes in service module.

                                        service
                                           |
                          -------------------------------------
                          |                                   |
               _GenericServiceManager                _SpecificServiceManager
                          |
            -----------------------------
            |                           |
      _SystemdServiceManager   _SystemdServiceManager

* ServiceManagerFactory provides two interfaces to create service.
    * create_specific_service_manager() to create _SpecificServiceManager.
    * create_generic_service_manager() to create _GenericServiceManager.
      * FactoryHelper provides a method get_service_manager_type() to
      auto detect the system type and return constructor of
      _SystemdServiceManager or _SystemdServiceManager.
      So, user do not have to know about the system type when they use
      create_generic_service_manager() to create _GenericServiceManager.

Outline of ServiceManagerFactory class:
                                             ServiceManagerFactory
                                                     |
            ---------------------------------------------------------------------------------------
            |                                        |                                            |
create_specific_service_manager()          create_generic_service_manager()                   FactoryHelper
            |                                        |                                            |
            |                                        |                      ----------------------------------------------
            |                                        |                      |                     |                      |
            |                                        |             |----------------------| |---------------------| get_name_of_init()
            |                                        |             |*get_specific_service_| |*get_generic_service_|
            |                                        |------------>|command_generator()   | |command_generator()  |
            |                                                      |*get_specific_service_| |*get_generic_service_|
            |                                                      |result_parser()       | |result_parser()      |
            |                                                      |----------------------| |*get_generic_service_|
            |------------------------------------------------------------------------------>|manager_type()       |
                                                                                            -----------------------

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
virttest.staging.service: minor fix for ServiceManagerFactory class.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>