oe-core recommends openssh-sftp-server since [1] this causes conflict
between openssh and dropbear when -tests image is building.
This happens because openssh-ptest is pulled in as complementary package
of openssh-sftp-server
Adding openssh to PACKAGE_EXCLUDE_COMPLEMENTARY excluded openssh from
the list. As a downside of this openssh-ptest,  -dev or -dbg packages
will not be installed using install_complementary and have to be
installed manually (IMAGE_INSTALL:append or similar).
Proper fix should go to oe-core (use -O flag to switch back to old scp
protocol if newer openssh server is used) however it's not clear if
openssh will keep this feature.

Upstream-Bug: https://bugzilla.yoctoproject.org/show_bug.cgi?id=14858

Fixes:
Error:
 Problem: problem with installed package dropbear-2020.81-r0.cortexa53_crypto
  - package dropbear-2020.81-r0.cortexa53_crypto conflicts with openssh provided by openssh-8.9p1-r0.cortexa53_crypto
  - package openssh-8.9p1-r0.cortexa53_crypto conflicts with dropbear provided by dropbear-2020.81-r0.cortexa53_crypto
  - package openssh-ptest-8.9p1-r0.cortexa53_crypto requires openssh, but none of the providers can be installed
  - conflicting requests
(try to add '--allowerasing' to command line to replace conflicting packages or '--skip-broken' to skip uninstallable packages)

[1] https://git.openembedded.org/openembedded-core/commit/?id=a98188e83b2c027d99cc38e3367e1ec2a98efbb0



Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>