Fix apt configure unittests to run in more environments

As well as some improvements that were found along testing them and due to
the fact that we review some of that code again in the scope of curtin
currently.

Tests:
 - add a test for an alternate keyserver
 - harden mirrorfail tests to detect and skip if no network is available
 - improve apt_source related tests to work on CentOS7

Changes:
 - gpg key handling is now in python instead of a shell blob and moved
   to its own module.
 - packages/bddeb has an option to sign as someone else than smoser
 - make exception handling of apt_source features more specific
   (do not catch broad 'Exception')
 - rename some functions to reflect better what they actually do
 - capture some helper subp calls output to avoid spilling into stdout when
   not intended

0 files changed, 0 insertions, 0 deletions