teuthology/suite: merge base_config with other fragments

teuthology/suite: merge base_config with other fragments

Presently the code tries to merge the base_config when the worker starts
running. There's no need to construct it this way and it prevents sharing the
"defaults" with the fragment merging infrastructure. It also prevents
overriding defaults like:

    kernel
        branch: wip-pdonnell-i66704
        client:
            branch: wip-pdonnell-i66704
            flavor: default
            kdb: 1
            sha1: 745cacd8f31e50d7f3b6039bbd8c9a8dfc07bf03
        flavor: default
        kdb: 1
        sha1: 745cacd8f31e50d7f3b6039bbd8c9a8dfc07bf03

A YAML fragment can set kernel.client but it cannot delete the defaults for
kernel.(branch|flavor|kdb|sha1) because there's no way to remove YAML elements
via a deep merge.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>