The setup step for Jenkins jobs was always running

pip install --user rbtools before applying a patch. If the workspace

within Jenkins already had a virtualenv this would fail entirely,

and even if not, it was wasteful to do it every time.
This now checks whether rbt is already on the PATH and only installs it

when needed. When it is missing, rbtools is installed into a virtualenv

in the workspace (reused across builds if present) rather than into the

user site directory.
The rbt patch invocation has also been reworked to pass the command as

an array instead of building a string and then splitting it on whitespace.

Created a Jenkins testbed and verified that the first time a Review

Board job was triggered, it created a virtualenv and installed RBTools,

and thereafeter it reused it.