bootstrap_minion and bootstrap_mon_ip handling could be improved

Currently, if the user sets both `bootstrap_minion` and `bootstrap_mon_ip`, the order in which they are set is important and can affect the outcome.

If the user sets `bootstrap_mon_ip` first, and `bootstrap_minion` second, BootstrapMinionHandler will clobber the user's `bootstrap_mon_ip` setting. It should preserve the user's setting in this case.

If the user sets `bootstrap_minion` first and `bootstrap_mon_ip` second, and the IP address specified is not an IP address of the bootstrap minion, ceph-salt will happily run `cephadm bootstrap` with these invalid options. Instead, ceph-salt should catch this in `validate_config` and raise an error.

Start by tracing BootstrapMinionHandler and validate_config, focusing on how bootstrap_minion and bootstrap_mon_ip are processed in each assignment order. Verify that an explicitly set bootstrap_mon_ip is preserved and that an address not belonging to the bootstrap minion causes validate_config to raise an error.

Written by the indexing model from the issue text.