iscsi related issues fixed

```
1. virttest.iscsi: Fix a regex and log level
    
    As the newly updated of iscsi related packages, the output of iscsi
    session is different from the past, so update the regex when
    searching iscsi device name to adapt the update.
    
    If no iscsi device, it should report error instead of debug msg.
    
2. 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  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>