[HAMMER] - Fixed display of worker memory threshold settings

[h-kataria]

Fixed couple of scenarios

• Updated code to use get_worker_setting method
• Worker memory threshold of 1 GB or above was not reflected on Workers tab because https://github.com/ManageIQ/manageiq-ui-classic/blob/master/app/controllers/ops_controller/settings/common.rb#L997 value of variable x needs to be converted to integer value from a float so value can be selected correctly in the drop down.
• If a value in yaml config file is set to as 600.megabytes, it is being returned as a string, so need to evaluate/convert the value so it can be correctly selected in the drop down.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1686901
Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1686902

(cherry picked from commit 6a20b57)

backport of #5189 @hstastna @himdel if one you can veirfy this fix on hammer, that will be great, i am having issues running hammer branch locally.

[hstastna]

I was able to setup Hammer and to test this PR and here is what I've found:

For https://bugzilla.redhat.com/show_bug.cgi?id=1656873,
I can see no change, it does not work well for Event Monitor, Refresh and Connection Broker, after saving the changes it shows the smallest values from the drop downs. Here is the screenshot after setting all the possible tresholds to 1.1 GB:

For https://bugzilla.redhat.com/show_bug.cgi?id=1658373,
it seems to work (tried more different values), the yaml file seems to change (Advanced tab) after changing the values in the UI, under Workers tab. Here is the screenshot after changing all the four values of tresholds (for Generic Workers, Priority Workers, C & U Data Collectors and C & U Data Processors) to 450 MB:

[miq-bot]

Checked commit h-kataria@142b265 with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 0 offenses detected
Everything looks fine. ⭐

[hstastna]

After some additional changes, it works well now. I approve this.