Keep logs in $BASE instead of $WORKSPACE

Avoid moving logs into the jenkins homedir unnecessarily, since it
has a tendency to fill up the root filesystem and the SCP publisher
can follow symlinks out of the workspace anyway. We've seen slaves
go offline due to full root filesystems recently, so this should
hopefully reduce the incidence of that problem.

Change-Id: I490ed53989137218f14464708a65969033c108d6
Closes-Bug: #1268732
Use /opt/git instead of workspace cache

Also, be better about removing cached things.

Change-Id: I7c8da7bc748ea67d7f97c56e9e2fbc2276d3051b
Merge "Keep logs in $BASE instead of $WORKSPACE"
Merge "Use /opt/git instead of workspace cache"