We have found that some users are unable to download the GMP plugin from
the default URL because of their specific network
configuration/circumstances. For example, some users cannot access the
*.gvt1.com domain. This patch adds the ability for balrog to supply
alternative mirror URLs to attempt to download the plugin, where we first
try the primary, and if supplied any secondaries, in the given order.
Original Revision: https://phabricator.services.mozilla.com/D229690
Differential Revision: https://phabricator.services.mozilla.com/D230617
