Multiple gpg keys for build cache

[simonpintarelli]

If the gpg file generated for the buildcache contains more than 1 key, pushing to the buildcache will fail:

spack -e ./gcc buildcache create --rebuild-index --only=package alpscache \
$(spack --color=never -e ./gcc find --format '{name};{/hash}' | grep -v -E '^();' | cut -d ';' -f2)
==> Error: Multiple keys available for signing
['<key1>', '<key2>']
Use spack buildcache create -k <key hash> to pick a key.
make[1]: *** [Makefile:16: gcc/generated/build_cache] Error 1
make[1]: Leaving directory '/dev/shm/<user>/gmx/compilers'
make: *** [Makefile:39: compilers] Error 2

TODO: make sure the gpg file contains only 1 key during stack-config stage