The remote backend uses backend.ParseVariableValues locally only to decide
if the user seems to be trying to use -var or -var-file options locally,
since those are not supported for the remote backend.
Other than detecting those, we don't actually have any need to use the
results of backend.ParseVariableValues, and so it's better for us to
ignore any errors it produces itself and prefer to just send a
potentially-invalid request to the remote system and let the remote system
be responsible for validating it.
This then avoids issues caused by the fact that when remote operations are
in use the local system does not have all of the required context: it
can't see which environment variables will be set in the remote execution
context nor which variables the remote system will set using its own
generated -var-file based on the workspace stored variables.
Properly wait for cost estimation to finish running before outputting
the results. Waits 500 milliseconds between checks, rather than backing
off exponentially, because we are not in a run queue. At the point we're
waiting, we expect cost estimation to be run in a timely manner.
This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.
To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.
