Implement and test a doCheckSauceConnectDirectory method that validates the plugin-level
"Sauce Connect Working Directory" configuration, ensuring it is either:
 * empty, in which case it (still) defaults to the jenkins user's home directory
 * is verified to be a directory that exists and is writable
Merge branch 'TRE-2149-prevent-misconfig-of-sauce-working-directory' into sfly-1.X

Resolves TRE-2149