Improve scripts for test suite download usc-tools

The robot scripts for donwloading usc-tools is complicated. Need to improve the robot scripts to be more clear for test users to understand. The suggestion is as follows:

${file_exist_status} Run Keyword And Ignore Error File Should Exist ~/usc-tools/UscAgent.tar
Run Keyword If "${file_exists_status}" == "FAIL" Run git clone ...

Also, the read until for "(17/17), done." is too specific and needs to be refactored to be generalized in case updates are made to the usc-tools repo.