allow governance body on parachain to have sovereign account (#1291)

The goal is to allow Fellowship on Collective chain to have a sovereign
account on Polkadot so that we can add it as an identity registrar. This
will allow Fellows origin to be able to provide judgements for
Fellowship members.

This currently allow any body on any parachain including non system
parachains to have sovereign account. I cannot think of any reason why
that may be an issue but let me know if I should change it to filter
only system parachains.

[This](https://gist.github.com/xlc/ec61bfa4e9f6d62da27d30141ad2c72b) is
the testing script.

Original PR: https://github.com/paritytech/polkadot/pull/7518