gluon-autoupdater: Refactor code.

Move building blocks of the update into seperate functions.
gluon-autoupdater: enable list of mirrors for manifest and firmware.

Instead of a single url this patch allows to have a list of urls. So in case
of network instability a router can still reach local mesh-cloud server to pull
a update from. In this stage the autoupdater simply tries every mirror until
one passes all tests.