Use an absolute path when mapping working folder

Certain versions of [Team Explorer Everywhere Command Line Client (TEE CLC)](http://www.microsoft.com/en-us/download/details.aspx?id=30661) do not support a relative folder (such as '.') when mapping the workfolder, but `tf.exe` does, leading to a broken workspace on systems with the latter, unless the absolute path to the workspace is specified in the Jenkins job (which introduces difficulties of its own).

This pull request simply obtains the absolute path to the workspace before performing the mapping, avoiding this feature mismatch between TEE CLC and `tf.exe`.