package_manager: support for signed DEB package feeds

["Ferry Toth" <fntoth@gmail.com>]

[PATCH v4 1/2] apt: add apt selftest to test signed package feeds
[PATCH v4 2/2] package_manager: fix missing dependency on gnupg when

Since Gatesgarth apt (1.8.2) has become more strict and doesn’t allow unsigned repositories by default.
Currently when building images this requirement is worked around by using [allow-insecure=yes] and
equivalently when performing selftest.
    
Patches "gpg-sign: Add parameters to gpg signature function" and "package_manager: sign deb package feeds"
(already in master) enable signed deb package feeds. When called from 
`oe-selftest -r runtime_test.TestImage.test_testimage_apt` this patch adds a runtime test for apt 
derived from the test_testimage_dnf test. It creates a signed deb package feed, runs a qemu 
image to install the key and performs some package management. To be able to install the key
the gnupg package is added to the testimage.

Changes in V4:
 - Add fix to make gnupg-native a dependency else hosttools is used and 
   `oe-selftest -r runtime_test.TestImage.test_testimage_apt` fails on Ubuntu 16.04 used 
   on the autobuilder (Alexandre Belloni)

Changes in V3:
 - When called from `bitbake core-image-sato -c testimage` package feed is unsigned. Auto-detect
   this case and behave as before (Richard Purdie)

Changes in V2:
 - Added runtime test for signed deb package feeds (Richard Purdie)