gpg: add 'spack gpg subcommand (#3845)

- Add a `spack gpg` subcommand in anticipation of signed binaries.
- GPG keys are stored in var/spack/gpg, and the spack gpg command manages them.
- Docs are included on the command.

var/spack/gpg/README.md

@@ -0,0 +1,5 @@
+# GPG Keys
+
+This directory contains keys that should be trusted by this installation of
+Spack.  They are imported when running `spack gpg init`, but may also be
+imported manually with `spack gpg trust path/to/key`.