Enhance iSCSI multipath support

Enhance iSCSI multipath support

Currently, nova-compute and brick support multipath for iSCSI volume
data path. It depends on response to targets discovery of the main
iSCSI portal, expecting multiple portal addresses to be contained.

However, some arrays only respond to discovery with a single portal
address, even if secondary portals are available. In this case,
the connector cannot know secondary portals and corresponding
iSCSI target IQN, so cannot establish multiple sessions for
the target(s). To enable the connector to login to secondary portals,
cinder should tell all the portal addresses and corresponding
target iqns/luns.

With this patch initialize_connection API will return connection_info
with multiple portal addresses/iqns/luns when multipath=True is
specified in the connector info. For example:

  {"driver_volume_type": "iscsi",
   "data": {"target_portals": ["10.0.2.15:3260", "10.0.3.15:3260"],
            "target_iqns": ["iqn.2010-10.org.example:target-01",
                            "iqn.2010-10.org.example:target-02"],
            "target_luns": [1, 2],
            ...}}

This also modify brick/iscsi/connector.py to make the parameter
recognized and to login to every target.

In addition, this adds an config option
'iscsi_secondary_ip_addresses' to specify a list of secondary
portal addresses for LVM iSCSI Driver.

Change-Id: I2a650c8ae70189b738937eaea7e5cd28f896257c
Implements: blueprint iscsi-multipath-enhancement