feature(sct_config): appendable configuration values

introduce new option for appending to string or list configuration values

1) strings: can be appended with adding `++` at the begining of the string:
   `export SCT_APPEND_SCYLLA_ARGS="++ --overprovisioned 1"`

2) list: can be appended by adding `++` as the first item of the list
   `export SCT_SCYLLA_D_OVERRIDES_FILES='["++", "extra_file/scylla.d/io.conf"]'`

Note list would work on every config defiend with `str_or_list_or_eval`
with the expection of `config_files`, `region_name`, `gce_datacenter` and `gce_datacenter`
which can't be appended, and would need to be overriden completly

Ref: #7653
(cherry picked from commit e80f2bf)