Bug 1410424 - [mozbuild] Add a 'quiet' argument to VirtualenvManager.install_pip_requirements r=mshal

Some requirements.txt are very large and result in a lot of package already
installed messages. Would be nice to hide this.

MozReview-Commit-ID: FQecuePM0zZ

tools/docs/mach_commands.py

@@ -51,7 +51,8 @@ class Documentation(MachCommandBase):
             return die('jsdoc not found - please install from npm.')
 
         self._activate_virtualenv()
-        self.virtualenv_manager.install_pip_requirements(os.path.join(here, 'requirements.txt'))
+        self.virtualenv_manager.install_pip_requirements(
+            os.path.join(here, 'requirements.txt'), quiet=True)
 
         import sphinx
         import webbrowser