Makefile: Allow to use another pip than the system one

When installing buildbot in a not-activated virtualenv, it comes handy to define PIP before calling make frontend.
Add possibility to restart single slave
Do not use --quiet options since it always return EXIT_SUCCESS
Add more information about buildbot upgrade errors (Refs: #2588)
Remove Progress/ETA support.

This is part of removing the reliance on status APIs in bug 2818.  The
progress interface wasn't very good, anyway, and will be re-implemented
in bug 2590.
Update tests
Merge pull request #1268 from djmitche/2818c

Remove Progress/ETA support.

See ticket:2818
Merge pull request #1266 from benallard/patch-6

Makefile: Allow to use another pip than the system one

See ticket:2877
Merge pull request #1267 from jollyroger/debian-fixes

Debian fixes