Use different algorithms depending on the ssh version

[artem-sidorenko]

Fixes GH-162

[artem-sidorenko]

PR to ssh-baseline follows..(and is the reason for WIP)

[artem-sidorenko]

Currently this feature is server only (basically the same implementation like with privilege_separation). There is an option HostKeyAlgorithms which is valid for server and client, however this option is present on the server starting from something like openssh 6.7 (I do not know the exact version). So a proper server&client implementation would require a bit more changes in order to handle this.

Below an overview over different ssh versions and supported things (key stuff is mostly based on the @atomic111 evaluation):

	ubuntu 12.04	ubuntu 14.04	ubuntu 16.04	centos 6.8	centos 7.3
ssh version	5.9	6.6	7.2	5.3	6.6
ssh -Q key support		x	x		x
HostKeyAlgorithms/server support			x
dsa key	x	x	x	x
rsa key	x	x	x	x	x
ecdsa key		x	x		x
ed25519 key		x	x		x

	debian 7.11	debian 8.6	fedora 24	fedora 25	opensuse leap 42.1	opensuse 13.2
ssh version	6.0	6.7	7.2	7.3	6.6	6.6
ssh -Q key support		x	x	x	x	x
HostKeyAlgorithms/server support			x	x	?	?
dsa key	x	x			x	x
rsa key	x	x	x	x	x	x
ecdsa key	x	x	x	x	x	x
ed25519 key		x	x	x	x	x

[artem-sidorenko]

and here is the PR with tests: dev-sec/ssh-baseline#79

[artem-sidorenko]

The tests for failing opensuse 42.1 are in PR dev-sec/ssh-baseline#84

[coveralls]

Coverage decreased (-0.8%) to 99.228% when pulling 34be301 on artem-sidorenko:alg-update into 5dfe85a on dev-sec:master.

[coveralls]

Coverage decreased (-0.4%) to 99.614% when pulling 98375b8 on artem-sidorenko:alg-update into 5dfe85a on dev-sec:master.

[coveralls]

Coverage remained the same at 100.0% when pulling f9baa14 on artem-sidorenko:alg-update into 5dfe85a on dev-sec:master.

[atomic111]

@artem-sidorenko thank you for the great work. now we can tag the version to 2.0.0. awesome