Backport iSCSI initiator name related fixes:
- Allow changing iSCSI initiator name after setting it
Resolves: rhbz#2221935
- Add a basic test case for the iscsi module
Resolves: rhbz#2221935
- tests: Use blivet-specific prefix for targetcli backing files
Resolves: rhbz#2221935
- iscsi: Save firmware initiator name to /etc/iscsi/initiatorname.iscsi
Resolves: rhbz#2221932
- tests: Improve iscsi_test.ISCSITestCase
Resolves: rhbz#2221935
