Do not overwrite role in install phase

This causes an exception later:

 undefined method `[]' for nil:NilClass
 /opt/dell/crowbar_framework/app/models/provisioner_service.rb:158
Merge pull request #189 from dirkmueller/fix_role_exception

https://github.com/crowbar/barclamp-provisioner/pull/189

Merged by devtool for VictorLowther