fix(sct_config): change type of perf_gradual_ params

[juliayakovlev]

The parameters 'perf_gradual_threads' and 'perf_gradual_throttle_steps' are defined with type 'dict'. It prevents dynamicaly changing them in the 'extra_environment_variables' parameter in Jenkins job (if we want to change them while running job, without push commit).
Changing the type to 'dict_or_str' allow us to change those params dynamically

Testing

• use extra_environment_variables

The 'perf_gradual_threads' and 'perf_gradual_throttle_steps' values are according to extra_environment_variables and config validation step did not fail

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

[fruch]

LGTM

[soyacz]

LGTM