Merging openstack_configuration sources can cause unhelpful errors

[geokala]

When trying to deploy a blueprint locally, I was receiving errors stating that the parameters I had supplied (in the blueprint) were not among the list of allowed parameter combinations.
It then listed the exact combination of parameters I had supplied among the acceptable options.

The actual cause was running with correctly supplied parameters in the blueprint, as well as having a sources openstackrc file with other parameters.

It would be nice if there were a way to prevent merging, e.g. an openstack_config flag in the blueprint that could force only specific configuration sources to be used (e.g. merging would occur as normal with the default value of:
openstack_config:
config_sources: [env_vars, file, blueprint]
but if only one (or two) sources were used then they would be the only ones used, e.g.
config_sources: [env_vars, blueprint]
or
config_sources: [blueprint]
)

Alternatively, for simplicity, just a way to specify only one config source would be helpful.

[EarthmanT]

#209 @geokala Would you like to review?