Fix the iSER legacy usage in the new targets flow

Fix the iSER legacy usage in the new targets flow

The iSER driver flow, after setting volume_driver to LVMISERDriver,
is broken in the new driver - target model in the master,
due to a mismatch in the target prefixes and a lack of iSER target
initialization, with the correct parameters.

In order to fix this:
  1. The function initialize_connection, that sets the iSER required
     parameters, was added to cinder/volume/targets/iser.py (and overrides the
     new connection initialization in cinder/volume/targets/tgt.py).
  2. The iSER and iSCSI prefixes equalized, due to the removal of brick/iscsi
     with the different default prefix for ISERTgtAdm (and for stability).

Closes-Bug:#1411470

Change-Id: Ie4b9b85b47e9da20a1de1e10415766413cd01f04