Test Bugfix: make BootSourceTest#doCheckImageIdWhenNoValueSet assertion locale indepent

[Farix1337]

If running the jenkins.plugins.openstack.compute.slaveopts.BootSourceTest#doCheckImageIdWhenNoValueSet at a machine without US or GB localisation the assertion will fail, because the created VALIDATION_REQUIRED which is used for the expect value changes based on the jvm default java.util.Locale (See the Implementation org.jvnet.localizer.ResourceBundleHolder#format)
This changes forces the US Localisation for the creation of the expected value of VALIDATION_REQUIRED.

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[Farix1337]

Currently the build is failing because plugins for a lower jenkins version than 2.235.4 can't be build.
I am working with @frkautz and @MintInfusion to fix this issue.
This Pull Request is just prep work for our next Pull Request to allow the usage of multifacor authenticated openstack apis.

[Farix1337]

Due to neceassry jenkins version update commits which poluted this Pull request i close this Pull Request and make a new one for updating the jenkins version: #331