Use github URL for repo manifest

Get the repo manifest from github for improved reliability and speed.
This needs the github changes on the stoq-manifest repository, or else it will
break things.

Change-Id: I82905b99562a9ead3a3b7369811f8f035a4bff05
Use the new stoq/tools/env.sh instead of _shell

_shell was moved to stoq/tools/env.sh. Use it instead of keeping the
duplicated code.

Change-Id: I3f864b9ee091c694e02c21feaddc8add5954da47