Enhance decorators in agent and automate creation of command_map

sync_command and async_command in ironic_python_agent/extensions/base.py
are implemented so that they change decorated method signature,
which is really bad practice. Example:

@sync_command
def orig_method(self, **kw):
...

sync_command decorator returns lambda which has one additional
argument 'command_name'

def sync_command(func):
    def decorated(self, command_name, **kw):
         ...
         func(self, **kw)
         ...
    return decorated

Additional parameter 'command_name' is moved from decorated function to
decorator signature in both decorators. Every call of decorated functions
is fixed, BaseAgentExtension.execute is updated to not send this extra
argument. BaseAgentExtension.__init__ automatically generates command_map
from decorated methods.

Change-Id: Iff0f495bc8d9c731892feb0f4759a7f10ee76328
Closes-Bug: 1314148
Merge "Enhance decorators in agent and automate creation of command_map"