[Ready for review] Make TOGGLE_DECOSPECS only add default decorator once.

[darinyu]

This is needed when we add value to DECOSPECS (or TOGGLE_DECOPSECS).

Changes:

1. update the METAFLOW_DECOSPECS (from config) to METAFLOW_DEFAULT_DECOSPECS. This will no longer collide with user override METAFLOW_DECOSPECS which translate to --with
2. move config_merge_cb from start to common_options. Reasoning:

Before the change:
when we use TOGGLE_DECOSPECS and run a flow, it first inject X as decorator at flow level (by this config_merge_cb. It then get passed to the subprocess when we run step. When step starts, it injects this X again.

After the change
when we use TOGGLE_DECOSPECS and run a flow, it no longer inject X (because this config_merge_cb is no longer in start in cli.py), When subprocess starts to run step, we inject this X to decorator (via common_options).