Introduces `PULUMI_HOME` environment variable which points to a path to the path to `.pulumi` folder. Defaults to `<user's home dir> + ".pulumi"` if not specified.
Fixes#2966. In addition to plugins, it "moves" the credentials file, templates, workspaces.
`bin` folder is intact: to move it, we need to adjust all installation scripts to respect `PULUMI_HOME` and put executables in the proper `bin` folder.