Update function parseRepoBranch()

[sophia-guo]

Originally the function parseRepoBranch() consume strings octocat/aqa-tests:test and return the repo octocat/aqa-tests and branch 'test'.
User will use the return repoBranch to generate the git url like --tkg_branch ${repoBranch[1]} --tkg_repo https://github.com/${repoBranch[0]}.git.

We'd like to enhance the parseRepoBranch() to include the git url generation logic.