Add tests to iscsi_deploy.build_deploy_ramdisk_options

The build_deploy_ramdisk_options() from the iscsi_deploy.py module had
no unittests, this patch is adding it.

Also this patch simplify a little the tests for
the pxe.build_pxe_config_options() by mocking
iscsi_deploy.build_deploy_ramdisk_options().

Change-Id: I7422027f8d65541cd4ca28b11df1d2a219152b88
Merge "Add tests to iscsi_deploy.build_deploy_ramdisk_options"