target/ssh.py: add HostKeyAlgorithms option to test commands

After recent updates to the autobuilder tumbleweed workers there are tests where the client and
server fail to agree on a public key algorithm for host authentication:

DEBUG: [Running]$ ssh -l root -o PubkeyAcceptedKeyTypes=+ssh-rsa -o UserKnownHostsFile=/dev/null
    -o StrictHostKeyChecking=no -o LogLevel=VERBOSE 192.168.7.6 export PATH=/usr/sbin:/sbin:/usr/bin:/bin; uname -a
DEBUG: time: 1634578090.4632802, endtime: 1634578390.4592378
DEBUG: Partial data from SSH call: Unable to negotiate with 192.168.7.6 port 22: no matching host key type found. Their offer: ssh-rsa

This appears to be an issue with recent versions of shh.  Add -o HostKeyAlgorithms=+ssh-rsa to
command invocation as suggested at:

http://www.openssh.com/legacy.html



Signed-off-by: Steve Sakoman <steve@sakoman.com>