Fix error changing ownership when deleting a user, if the current user has temporary credentials

[sworisbreathing]

When deleting a user, the provider attempts to chown all of the user's objects to $REDSHIFT_USER. If you use temporary credentials provided by GetClusterCredentials the username you need to authenticate as is prefixed with either IAM: or IAMA:. However, the username needed for the chown is the non-prefixed one.

This PR fixes the issue by stripping these specific prefixes from the chown statements (and also parameterizes them).

You can reproduce the error like so:

username=<your db username>
clusterIdentifier=<redshift cluster identifier>
credentials="$(aws redshift get-cluster-credentials --db-user=$username --cluster-identifier=$clusterIdentifier)"

# This assumes you have jq on your PATH. otherwise just "echo $credentials" and set the values explicitly
export REDSHIFT_USER="$(echo $credentials | jq -r .DbUser)"
export REDSHIFT_PASSWORD="$(echo $credentials | jq -r .DbPassword)"

make testacc

Note: while this PR is in a similar vein to #3 the underlying issue isn't necessarily dependent upon the changes there.