allow customization of apt-get command, add --force-unsafe-io

This allows the customization of the apt-get command used for installing
packages, and also adds '--force-unsafe-io'.  Because this is spawned from
cloud-init, it seems to make sense as a first boot package installation
option.

ChangeLog

@@ -47,6 +47,7 @@
  - upstart: cloud-init-nonet.conf trap the TERM signal, so that dmesg or other
    output does not get a 'killed by TERM signal' message.
  - support resizing partitions via growpart or parted (LP: #1136936)
+ - allow specifying apt-get command in distro config ('apt_get_command')
 
 0.7.1:
  - sysvinit: fix missing dependency in cloud-init job for RHEL 5.6 

cloudinit/distros/debian.py

@@ -33,6 +33,10 @@ from cloudinit.settings import PER_INSTANCE
 
 LOG = logging.getLogger(__name__)
 
+APT_GET_COMMAND = ('apt-get', '--option=Dpkg::Options::=--force-confold',
+                   '--option=Dpkg::options::=--force-unsafe-io',
+                   '--assume-yes', '--quiet')
+
 
 class Distro(distros.Distro):
     hostname_conf_fn = "/etc/hostname"
@@ -150,8 +154,7 @@ class Distro(distros.Distro):
         # See: http://tiny.cc/kg91fw
         # Or: http://tiny.cc/mh91fw
         e['DEBIAN_FRONTEND'] = 'noninteractive'
-        cmd = ['apt-get', '--option', 'Dpkg::Options::=--force-confold',
-               '--assume-yes', '--quiet']
+        cmd = list(self.get_option("apt_get_command", APT_GET_COMMAND))
 
         if args and isinstance(args, str):
             cmd.append(args)