Alliance pallet: add force_set_members instead of init_members function

[muharem]

Fixes https://github.com/paritytech/substrate/issues/11928

Root call init_members which is mean to initialize the alliance can be called only once. If an initial call had a mistake or the alliance lost the trust, there is no way to reset the alliance.

In the PR we changing the name and behaviour of the init_members function. Instead force_set_members function lets perform a root call on already initialized alliance, which will reset it and initialize new.