Add the iSCSI dependency packages to README

The README file contains instructions to build Kimchi from the source
tree, including the exact commands (e.g. yum, apt-get, zypper) to install the
dependencies on the main Linux distros. But none of those commands contain the
iSCSI package which is needed to validate some types of storage pool devices.

Add the iSCSI package ("iscsi-initiator-utils" or "open-iscsi", depending on the
distro) to the install commands in the README file.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
UI bug fix: Set vlan_id value only if 'Enable VLAN' is checked

Commit 2f54aaba added a checkbox to provide choice of enabling vlan tagging
while creating a bridged network, but it doesn't check the checkbox before
setting the vlan_id to send server request.

Because that when the user tries to create a bridged network without enabling
VLAN the following error is raised:
Invalid parameter: 'None is not of type u'integer''

Fix it by setting vlan_id only if the vlan checkbox is checked.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>