Make destroy method for unmanaged deployments configurable

Sometimes there might be a need to not do a full wipe of all metadata in unmanaged builds (deployCI).  A new environment variable, UNMANAGED_DESTROY_MODE, can be set to anything other than `full` which will use destroyStaleMetadata instead of uninstall to incrementally delete metadata rather than do a full uninstall at the start of every build.

NOTE: Using destroyStaleMetadata in deployCI can cause build failures if building two different branches with different version.properties requirements.