use same master eval logic for Minion and SMinion

This fixes saltstack#24354
There was inconsistent behavior between salt-minion (`Minion` under the hood)
and salt-call (`SMinion` under the hood) with regard to how the master was
determined from the minion config.  `SMinion` had its own hard-coded master
evaluation in its `__init__` method that was not congruent with documentation.
The "standard" master evaluation logic existed in `master_eval`, a method of
`Minion`.
This was resolved by moving `master_eval` into `MinionBase` (which `Minion`
inherits from) and making `SMinion` also subclass `MinionBase`, which
otherwise only consists of a couple of methods that `SMinion` won't access.