Avoid Timed out error when umask is 027 or 077

If LXC commands are run with a restrictive umask like 027 or 077, then
the root directory of new containers will lack read `r` and access `x`
permission for non-root users.  The first failure to result from this
during `vagrant up` is that the SSH daemon cannot read the crucial file
`/home/vagrant/.ssh/authorized_keys` after it drops privileges to the
level of the `vagrant` user.  The result is the familiar:

```
    default: Warning: Authentication failure. Retrying...
    default: Warning: Authentication failure. Retrying...
Timed out while waiting for the machine to boot. This means that
Vagrant was unable to communicate with the guest machine within
the configured ("config.vm.boot_timeout" value) time period.
```

So we should make sure that we run all LXC commands with a umask that at
least does not prevent group and world `r` and `x` bits from being set
in newly created files and directories.

lib/vagrant-lxc/sudo_wrapper.rb

@@ -13,12 +13,25 @@ module Vagrant
 
       def run(*command)
         options = command.last.is_a?(Hash) ? command.last : {}
+
+        # Avoid running LXC commands with a restrictive umask.
+        # Otherwise disasters occur, like the container root directory
+        # having permissions `rwxr-x---` which prevents the `vagrant`
+        # user from accessing its own home directory; among other
+        # problems, SSH cannot then read `authorized_keys`!
+        old_mask = File.umask
+        File.umask(old_mask & 022)  # allow all `r` and `x` bits
+
+        begin
           if @wrapper_path && !options[:no_wrapper]
             command.unshift @wrapper_path
             execute *(['sudo'] + command)
           else
             execute *(['sudo', '/usr/bin/env'] + command)
           end
+        ensure
+          File.umask(old_mask)
+        end
       end
 
       private