Fix solaris_execute in SolarisISCSIDriver.

Fix solaris_execute in SolarisISCSIDriver.

A previous refactor [1] of SolarisISCSIDriver and SanDriver renamed
the `_execute` function to `san_execute`. However, SolarisISCSIDriver
still called the _execute function, which no longer exists in it's
parent classes.

This change calls san_execute instead of _execute in
super(SolarisISCSIDriver, san).

[1] https://review.openstack.org/#/c/38194/

Closes-Bug: #1318108
Change-Id: I4cafefdb3eef8f3c2c345907d7eabd4e8f88ef65
(cherry picked from commit a7f24f0a2395594ead7eb8a5cef894b257cd7e4f)