[V2V] Throttler - Replace class constant with global setting

[ghost]

ManageIQ/manageiq#18528 introduced new keys in Settings.transformation.limits.max_concurrent_tasks_per_ems. This PR updates the migration throttler to use it instead of a class constant.

Associated RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1688951

[ghost]

@miq-bot add_label transformation, enhancement, hammer/yes
@miq-bot add_reviewer @djberg96
@miq-bot add_reviewer @jameswnl
@miq-bot add_reviewer @agrare

[miq-bot]

Checked commit fabiendupont@e62d533 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. ⭐

[agrare]

@fdupont-redhat this seems unrelated to the conversion host BZ

[ghost]

@agrare It doesn't have to be backported. Wrong copy/paste of labels.

[ghost]

@miq-bot remove_label hammer/yes

[mturley]

@fdupont-redhat @jameswnl I think this PR needed to be hammer/yes because it consumes the setting added in #18528 and added to the UI in ManageIQ/manageiq-v2v#897, which were both hammer/yes. I'm not sure which BZ it should be attached to, though. It's causing problems for QE in verifying https://bugzilla.redhat.com/show_bug.cgi?id=1693746

[jameswnl]

I am a little outdated as to whether the major throttling rework pr is backed ported or not (It is backported, but is the backport reverted later?)
If that is back ported, then yes, this PR should be as well.

[mturley]

If this one is not to be backported, we should revert the UI change in ManageIQ/manageiq-v2v#897. As it stands right now the UI setting for max migrations per provider has no effect (on hammer).

Looking through the commit log at https://github.com/ManageIQ/manageiq/commits/hammer, I see #18415 backported on March 8, and I don't see that PR number mentioned in any later commits. Seems backported to me?

[mturley]

@miq-bot add_label hammer/yes

[simaishi]

Hammer backport details:

$ git log -1
commit 59d5d163e7e945518d0aef81734f7574d48fe1c9
Author: Adam Grare <agrare@redhat.com>
Date:   Mon Mar 11 09:25:34 2019 -0400

    Merge pull request #18539 from fdupont-redhat/v2v_use_settings_for_max_concurrent_tasks_per_ems
    
    [V2V] Throttler - Replace class constant with global setting
    
    (cherry picked from commit 5b6e5c56e76e3d928846a3f4078c84dcf1804bc5)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1693746