Remove dependency on ruby-wmi

This backports
https://github.com/opscode-cookbooks/jenkins/commit/d30b2bcaaef902c8e3367f1ed2cb1148da26ac73
to the 1.2.3 cookbook.  It eliminates the the dependency on ruby-wmi
since Chef stopped shipping this gem by default.
Convert Windows workspace to use backslash paths
Allow specifying an explicit node name
Always use the given home attribute if passed
Allow specifying cli url/home to jenkins_node