plugin: fix dynamic import mechanism

The current kimchi plugin subsystem implement a dynamic discovery and
import mechanism. It uses the Python builtin function __import__() to
actually import a plugin module by name. However it turns out it only
works when we start kimchi from the project source directory, not work
when it is started from the installed path.

This is because the __import__() accepts a "global" arugment and it
needs "__name__", "__path__" and "__package__" in this "global" dict to
determine the package context [1][2]. When in kimchi/utils.py it runs
"import plugins.XXX", it actually fetch the "__package__" from utils.py,
which is "kimchi", then it imports "kimchi.plugins.XXX" under the hood
[3].

The current import_module() function calls __import__() without giving a
global dict. When kimchid is started from /usr/bin, without the package
context, __import__() can not find "plugins.sample".

This patch provides the full package context to __import__() to mimic
the standard "import" statement behavior.

[1] http://docs.python.org/2/library/functions.html#__import__
[2] http://hg.python.org/cpython/file/990d7647ea51/Python/import.c
[3] http://docs.python.org/2/reference/simple_stmts.html#import

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>