rework package mirror selection

There are several changes here.
 * Datasource now has a 'availability_zone' getter.
 * get_package_mirror_info
   * Datasource convenience 'get_package_mirror_info' that calls
     the configured distro, and passes it the availability-zone
   * distro has a get_package_mirror_info method
   * get_package_mirror_info returns a dict that of name:mirror
     this is to facilitate use of 'security' and 'primary' archive.
   * this supports searching based on templates. Any template
     that references undefined values is skipped. These templates
     can contain 'availability_zone' (LP: #1037727)
   * distro's mirrors can be arch specific (LP: #1028501)
   * 'ec2_region' substitution variable is present only if this
     appears (by availability-zone naming convention) to be in EC2.
 * rename_apt_lists supports the "mirror_info" rather than single mirror
 * generate_sources_list supports mirror_info, and as a result, the
   ubuntu mirrors reference '$security' rather than security (LP: #1006963)
 * remove the DataSourceEc2 specific mirror selection, but instead
   rely on the above filtering, and the fact that 'ec2_region' is only
   defined if the availability_zone looks like a ec2 az.
 * package_mirrors is now required in the system_info config, a dict like:
   package_mirrors:
     - arches: [i386, amd64]
       failsafe:
         primary: http://archive.ubuntu.com/ubuntu
         security: http://security.ubuntu.com/ubuntu
       search:
         primary:
           - http://%(ec2_region)s.ec2.archive.ubuntu.com/ubuntu/
           - http://%(availability_zone)s.clouds.archive.ubuntu.com/ubuntu/
         security: []
     - arches: [armhf, armel, default]
       failsafe:
         primary: http://ports.ubuntu.com/ubuntu
         security: http://ports.ubuntu.com/ubuntu

LP: #1006963, #1028501, #1037727

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