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
change step state_strings to state_string: DB model
change step state_strings to state_string: DB API
change step state_strings to state_string: data API + consumers
remove outdated TODO
change build state_strings to state_string: DB model
change build state_strings to state_string: DB API
change build state_strings to state_string: Data API + consumers
add a --no-js option to validate.sh
switch $slow to $quick and negate
Add more information about buildbot upgrade errors (Refs: #2588)
Update tests
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
Merge pull request #1270 from djmitche/validate-no-js

Add `--no-js` option to validate.sh
angular 'date' filter expects dates to be in milliseconds.
Merge pull request #1274 from benallard/patch-6

angular 'date' filter expects dates to be in milliseconds.

Fixes ticket:2952
update docs to specify right type for state_string