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)
* 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.
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