Fix octavia-tempest-plugin on old tempest releases

This is a follow-up commit for 'Add type to allow ECDSA keys'
(I34ac429ab5442cef056ee8b63fcb2ba41e8b9b27), this commit allows
using octavia-tempest-plugin with older tempest releases
(ex: in our jobs on stable/train).
It catches the exception if [validation].ssh_key_type is not
supported and doesn't pass the ssh_key_type argument to tempest
functions.

Change-Id: I5c2db87975803b30ea230c3bbf5dab4b96da4614
2 files changed