Various changes for 0.6.16

[jwatson0]

• slm_builder/conan_lbstanza_generator: 0.6.16 - correct ordering of openssl libs
• Makefile: use windows-style venv if available
• slm_builder/conanfile.py: require lbstanzagenerator_pyreq >=0.6.16 <0.7.0
• slm_builder/conan_lbstanza_generator: add README.md documenting lbstanzagenerator_pyreq upload
• Remove root-level conan_lbstanza_generator, use the one from conan artifactory
• Temporarily disable tests/file-utils.stanza test
• Rename test build target and copy test files during build
• Use stanza 0.18.94
• Release 0.6.16