install.sh: quiet apt install for Vagrant

Vagrant VM provisioning doesn't have any mechanism for user interaction.
For this reason, it is necessary to tell APT that we are in a
non-interactive session and any install question should be answered
automatically by its preset.

For this reason, according to issue #42 and GitHub PR #43, I propose to
add "quiet" option to install.sh. This option tells Debian-based systems
to run APT in non-interactive mode. In case of important questions
during install, we can update the preset list specified in quiet_install
subroutine.

Flag functionality can always be extended in the future to support
unexpected cases where Vagrant provision hangs due to user interaction
request by some other program.

Change-Id: Iab3ce5f9b19cd6bfcf2fa180c433fb5d5d521914
diff --git a/docs/install.rst b/docs/install.rst
index 391fdfa..9073622 100644
--- a/docs/install.rst
+++ b/docs/install.rst
@@ -67,6 +67,15 @@
 
     ./install.sh -mni
 
+To install in "quiet" mode (without user interaction), use:
+
+::
+    ./install.sh -qa
+
+.. note::
+    The order of the flag -q is important to ensure that the environment is ready for
+    a quiet install.
+
 See ``install.sh -h`` for detailed options.
 
 Installing Dependencies