Allow executing jf from system path

[yahavi]

• [x] This pull request is created in the jfrog/jenkins-jfrog-plugin repository.

Fix #33

If no 'jfrog' tool was provided, try to run 'jf' (or 'jf.exe') from the system path. To be more clear, log also the following message:

A 'jfrog' tool was not set. Using JFrog CLI from the system path.