522d9e260b
Allow the following formats:
```yaml
mirrors:
name: <url>
```
```yaml
mirrors:
name:
url: s3://xyz
access_pair: [x, y]
```
```yaml
mirrors:
name:
fetch: http://xyz
push:
url: s3://xyz
access_pair: [x, y]
```
And reserve two new properties to indicate the mirror type (e.g.
mirror.spack.io is a source mirror, not a binary cache)
```yaml
mirrors:
spack-public:
source: true
binary: false
url: https://mirror.spack.io
```