Add clarification for jenkins execution module
Add clarification for jenkins execution module

There are two modules on PyPI which import as "jenkins". Installing the
wrong one will result in a traceback when the execution module tries to
use the Python bindings. This commit adds information about the
dependency to the docstring for the module. It also checks to make sure
that jenkins.Jenkins is present in the __virtual__() func, and if not it
will fail to load the execution module with a meaningful error message
in the log.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt