Mach Bootstrap: Support for more platforms

[UK992]

Adds support for Debian, Fedora and OS X.
On osx you need to choose package manager (Homebrew or MacPorts). Chosen manager will be set as default manager and written in .servo/default_package_manager file. If only one package manager exist, will be automatically chosen as default.

Also add new flag --silent and checks for missing packages before every build on all supported platforms.

If pip or virtualenv missing, it will run script to install packages needed for creating virtualenv.

Still need some testings if it works as expected, especially osx which i could only tested via travis-ci and at half-working virtual machine.

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This change is

[highfive]

Heads up! This PR modifies the following files:

• @wafflespeanut: python/servo/bootstrapper/packages.py, python/servo/bootstrapper/bootstrap.py, python/servo/command_base.py, python/servo/bootstrapper/windows_gnu.py, python/servo/bootstrapper/debian.py, python/servo/bootstrapper/osx.py, python/servo/bootstrapper/centosfedora.py, python/servo/bootstrapper/windows_msvc.py, python/servo/bootstrapper/base.py, python/mach_bootstrap.py, python/servo/bootstrap_commands.py

[aneeshusa]

I think we can re-use/migrate our Salt configurations from saltfs here instead of re-creating the package maangement logic, which I would prefer to do. I will check into the feasibility of this and leave another comment.

[UK992]

@aneeshusa Any informations about feasibility of Salt configuration?

[aneeshusa]

@UK992 It looks like Salt can be run from inside a virtualenv, so I'm working on a PR that sets that up.

[larsbergstrom]

This is SUPER exciting to me because it means that user machines and CI machines are provisioned in exactly the same manner. Thank you both for working on it!

[aneeshusa]

FYI, this is on hold until servo/saltfs#483 is merged + deployed + the kinks are worked out, since it jumps our Salt version by two major versions (to the latest).

[bors-servo]

☔ The latest upstream changes (presumably #13382) made this pull request unmergeable. Please resolve the merge conflicts.

[wafflespeanut]

Ping @aneeshusa

[aneeshusa]

Watch #14974 for updates.

[aneeshusa]

Closing since #14974 is merged, however @UK992 I'd love to see the Fedora/Debian support added to the Salt-based mach bootstrap by upstreaming those package lists to the saltfs repo!