task/install: extra_system_packages at the top level can be a list

task/install: extra_system_packages at the top level can be a list

Commit fc5a3197f22e ("task/install: allow both list and dicts for
extra_system_packages") introduced a regression for job definitions
where extra_system_packages at the top level (i.e. on the install
task itself) is a list.  For example

  overrides:
    install:
      ceph:
        flavor: default
        sha1: b7d57fefbec68934ae5e6e7d7c48ac09182397a5
      extra_system_packages:
      - pv

produces AssertionError on

  assert isinstance(b, dict)

in deep_merge() when it's called from _override_extra_system_packages().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>