Archiving / recording site build info

[cjlegg]

Record specs / envs used somewhere. Candidates are the provenance/ sibiling to each spack site or the archive folder in teh spacksites repo.

[cjlegg]

Note as yet unused .ini key under [spack_env_templates]

[cjlegg]

Note self.provenance is set in Site.init() but is not yet used for anything - it is there for the purpose of this issue