Extract cluster related logical units from cmdlib

All LUCluster* classes are extracted to cluster.py. Shared functions are
extracted to common.py, helper functions only used by LUCluster* are
extracted to cluster.py.
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>

Makefile.am

@@ -311,6 +311,7 @@ cmdlib_PYTHON = \
 	lib/cmdlib/__init__.py \
 	lib/cmdlib/common.py \
 	lib/cmdlib/base.py \
+	lib/cmdlib/cluster.py \
 	lib/cmdlib/tags.py \
 	lib/cmdlib/network.py \
 	lib/cmdlib/test.py