Change format of some judgments

Because "not volume" will be True if volume id is 0.It can not
distinguish 0 from None.The same to some other judgments where
variable value may be 0.

This patch also does the following changes:
Deletes check storagepool in function _check_conf_file and adds
check default iscsi target IP in function initialize_connection.
Moves create host group from do_setup to initialize_connection.
Changes _get_login_info to be _update_login_info.

Fixes bug: 1160340
Change-Id: I0434b587d57c783f10d5724c4c66aea5df0b32e5
Add du to rootwrap filters

Setting "nfs_disk_util=du" or "glusterfs_disk_util=du" will use
du instead of df for space calculation, but du is not allowed by
rootwrap.

Change-Id: I35522d40f97ea14cacf2637603f06a15810a0b36
Merge "Change format of some judgments"
Merge "Add du to rootwrap filters"