Add 'alt_arch' capability to apt_mirror::mirror and mirror.erb template 

The 'alt_arch' variable is an optional array to allow the mirroring of packages for alternative architectures in addition to the default architecture defined in apt_mirror::init.

e.g. Produces the following **additional lines** in 'mirrors.list'
\# mirror puppetlabs-trusty
deb http://apt.puppetlabs.com/ trusty main dependencies
**deb-i386 http://apt.puppetlabs.com/ trusty main dependencies**
**deb-armel http://apt.puppetlabs.com/ trusty main dependencies**

deb-src http://apt.puppetlabs.com/ trusty main dependencies