TgtAdm: Don't change CHAP username/password on live migration

TgtAdm: Don't change CHAP username/password on live migration

As tgtd doesn't update CHAP username/password while the initiator is
connected, CHAP username/password must not be changed while a Nova
instance are performing live-migration; otherwise the compute node
which the instance migrates to cannot login to the volume and the
migration process is aborted.

This fixes TgtAdm implementation not to regenerate random
username/password every time initialize_connection is called.
Also, it enables CHAP auth in unit tests of TargetAdmin helpers.

Change-Id: I48729a33fada62a7c8e4fe500b1e39533d898701
Closes-Bug: #1383509
(cherry picked from commit c22038b9005070e51224f5057aac9f73cf4d0340)