deploy_partition_image method may change root_mb value if image does not fit it

submitted patch: https://review.openstack.org/#/c/179043/

Reviewed: https://review.openstack.org/179043
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=c2924949c1e9a659acaf9da7be16d8fa176c0ad2
Submitter: Jenkins
Branch: master

commit c2924949c1e9a659acaf9da7be16d8fa176c0ad2
Author: Vladyslav Drok <email address hidden>
Date: Thu Apr 30 15:30:11 2015 +0300

    Improve root partition size check in deploy_partition_image

    There is a check in deploy_partition_image which ensures that size of
    the image being deployed is greater than or equal to root partition
    size. It is being checked on driver.deploy step by calling
    iscsi_deploy.check_image_size by in-tree drivers, but if some
    out-of-tree driver would use this function without calling
    iscsi_deploy.check_image_size beforehand the logic would be
    incorrect, as deploy_partition_image will silently change root_mb
    value and proceed.

    In this change it is proposed to throw an exception instead in this
    case.

    Closes-bug: #1454263

    Change-Id: Iecd59d938762089ab639066fa994492a64e92937