host_key_files should not include ssh_host_ecdsa_key on every host

[bkw]

centos-6.5 ships with openssh-5.3p1, but ecdsa keys are only supported since 5.7.
This results in an error 'Could not load host key: /etc/ssh/ssh_host_ecdsa_key' in syslog for every login.

We'll probably have to make the values of host_key_files distro specific.

[bkw]

... turns out the system was missing a yum update, after that, even openssh-server-5.3p1-104.el6_6.1.x86_64 supports ecdsa keys. I'll close this.