setup_or_cleanup_iscsi: Robust assumptions and pep8 fix

When reviewing jferlan's PR(tp-libvirt#47) to fix vol_create_from case,
i found the problems also exist in setup_or_cleanup_iscsi(), so pick up
the fixes:

When setup iscsi env, the function assumed that the iscsi tools
'tgtadm' and 'iscsiadm' were present on the host which may cause
all sorts of errors and failures. So before running these commands,
make sure the tools exist.

Additionally within iscsi_login, extended testing found that the SCSI
/dev/sdxx device didn't instantaneously appear. Sometimes it took an
extra second or two to appear, so added a loop for get_device_name().

Fix a wrong selinux status, the acceptable values are
['enforcing', 'permissive', 'disabled']

Meanwhile, a pep8 fix to make sure no more combined module imports.

Signed-off-by: Yanbing Du <ydu@redhat.com>