Fixing build parameters not being expanded in slave client name format

[stuartrowe]

swapping order of param substitution when getting effective client name so build parameters put in the slave client name format are substituted.

[cloudbees-pull-request-builder]

plugins » perforce-plugin #102 SUCCESS
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[jenkinsadmin]

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[oleg-nenashev]

Exposure of build parameters to client name is hazardous, because Perforce Plugin won't be able to handle variables during the polling, workspace cleanup, etc.

Would you like to create a JIRA issue for the case and then submit the PR to https://github.com/jenkinsci/perforce-plugin/compare/variables_handling_refactoring ? I'll merge your change together with the updated variables resolver

[rpetti]

Exposure of build parameters to the client name is already being done and is a widely used feature. During polling, the parameter default is used. This pull request simple allows parameters in the 'client name format for slaves' option to also be resolved, which was admittedly an oversight on my part.

[oleg-nenashev]

I agree that the PR should be somehow merged. I've just mentioned possible (and well-known) downsides.

[oleg-nenashev]

Merged the PR into #54