envsetup: Add CMGerrit 3.0

Auto-detects CM Gerrit username (based on repo init)
Auto-detects current repo (requires being in root of the repo)
Cleaned up code and better error checking

Change-Id: I139eff60824bae3c225bdf80a3d24c774e62dfa2
Change-Id: Ib8702ae06898813861ee426cec88513aa78bff13
Change-Id: Idfd85b19fdca018c4a744ee407c2659872192f55