Add support for iSCSI discovery

[bmeagherix]

Add support for iSCSI discovery

This uses iscsi_discovery_sync which was added to libiscsi in version 1.18.0 (Oct 9 2016) in commit sahlberg/libiscsi@d3ef192

Have also added CHAP and Mutual CHAP support by plumbing iscsi_set_initiator_username_pwd and iscsi_set_target_username_pwd. The former is available since version 1.1.0 (commit sahlberg/libiscsi@40abe84), whereas the latter was added in version 1.14.0 (commit sahlberg/libiscsi@b1d0ac4)

Example usage:

import iscsi
import pprint
ctx = iscsi.Context('iqn.1993-08.org.debian:01:1da68ca3c4ec')
ctx.set_session_type(iscsi.ISCSI_SESSION_DISCOVERY)
ctx.set_header_digest(iscsi.ISCSI_HEADER_DIGEST_NONE)
# ctx.set_initiator_username_pwd("user1", "12charsecret")
# ctx.set_target_username_pwd("user2", "13charsecret1")
ctx.connect('192.168.100.101', -1)
result = ctx.discover()
ctx.disconnect()
pprint.pprint(result)