Improve how mirrors are used in gitlab ci, where we have until now thought
of them as only a string.

By configuring ci mirrors ahead of time using the proposed mirror templates,
and by taking advantage of the expressiveness that spack now has for mirrors,
this PR will allow us to easily switch the protocol/url we use for fetching
binary dependencies.

This change also deprecates some gitlab functionality and marks it for
removal in Spack 0.23:

    - arguments to "spack ci generate":
        * --buildcache-destination
        * --copy-to
    - gitlab configuration options:
        * enable-artifacts-buildcache
        * temporary-storage-url-prefix