Change default parameters from mutable type into immutable.

Change default parameters from mutable type into immutable.

Get ride of these pylint warnings:

W0102:190,0:cleanup_chrootenv: Dangerous default value [] as argument
W0102: 47,4:ImagerPlugin.check_image_exists: Dangerous default value [] as argument
W0102:109,8:read_kickstart.KSHandlers.__init__: Dangerous default value {} as argument
W0102:708,0:get_repos: Dangerous default value {} as argument
W0102:772,0:get_attachment: Dangerous default value [] as argument
W0102:775,0:get_pre_packages: Dangerous default value [] as argument
W0102:778,0:get_packages: Dangerous default value [] as argument
W0102:781,0:get_groups: Dangerous default value [] as argument
W0102:784,0:get_excluded: Dangerous default value [] as argument
W0102:787,0:get_partitions: Dangerous default value [] as argument
W0102:877,4:BaseImageCreator.install: Dangerous default value {} as argument
W0102:210,4:Bootstrap.create: Dangerous default value [] as argument

Change-Id: Icc04896829ecfb0e2542f4d4755f4adb07b01319