[Merge] lp:~nacc/ubuntu-seeds/ubuntu.xenial into lp:~ubuntu-core-dev/ubuntu-seeds/ubuntu.xenial

[Nish Aravamudan]

Promote PHP7.0 to main by using the php-defaults provided versions of all PHP packages.
Prepare for the removal of PHP5 from the archive by removing it from the seeds.
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https://code.launchpad.net/~nacc/ubuntu-seeds/ubuntu.xenial/+merge/285932
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