[SRU ALL] LP#1535634 -- linux-signed: does not build in a private PPA

[Andy Whitcroft]

When linux-signed builds in a PPA the URLs for the PPA pool contain
authentication information.  Handle this so that linux-signed will build
correctly in a private PPA.  Following this email is a patch for all
existing linux-signed packages.  This is already applied and tested on
xenial linux-signed.

Proposing for application to all branches of linux-signed.

-apw