[iec] seba-on-arm: Installer/OS specific params

The IEC usecase job (e.g. seba-on-arm installation) parameters should
be installer specific and OS specific, e.g. Ubuntu deployed using Fuel
defaults to 'ubuntu' user, while CentOS deployed using Fuel defaults
to 'centos'.

While at it, unify currently separate parameters for the SSH
credentials in Fuel/Compass installations and fix the IEC dir location
for Fuel.

Change-Id: I93613aacc41eb8a2cfd1662b7d76ade36c7c8090
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>