Use env variables as tripleo-cd cloud params

Currently, tripleo-cd element runs devtest-overcloud.sh script
passing all parameters used as positional arguments. Given the
fact that we already have 9 of them, it would be much more
readable and less error-prone to pass them using env variables
instead (additionally, this would allow us to use those params
in hand execute version of devtest guide).

Backwards compatibility is retained by accepting parameters both
as positional arguments and environment variables. Positional
arguments take precedence over environment variables.

Change-Id: Ieb3e9373c6f4364b8c83ac4ebe19268957c770c1