There are 2 ways to define variables for substitution:
1) Define them as Environment variables and set EnvVars: true
in clusterctl object
2) Define them in additional-vars map in clusterctl object
Also adds possibility not to substitute variables if they are not defined in
in additional-vars or in environment for specific provider. But be aware,
if these variables are defined they will be substituted even if
variable-substitution: true
Change-Id: I0c92b3c37ac7b2e7c48c1033c074baef48f752a7
Relates-To: #284
a4107e7f1d