install.upgrade: deepcopy() overrides before we modify it

Since we modify the dict, make sure it is a deep copy.
Otherwise, a sequence like:

- install.upgrade:
    all:
      branch: dumpling
...
- install.upgrade:
    all: null

will delete the overrides keys in the first upgrade call and
then not have them at all in the second.

Signed-off-by: Sage Weil <sage@inktank.com>