tests: fix bgp-auth topotest

tests: fix bgp-auth topotest

Individual tests must not depend on each other. In particular, a test
can't be sure that the previous test config is applied or cleared.
It is definitely not true when a single test is executed, for example:
`test_bgp_auth.py::test_prefix_peer_remove_passwords`.

This commit makes all tests independent of each other. It also adds a
call to check_all_peers_established at the start of "remove_passwords"
tests to make sure that we not only block new peers with an incorrect
password, but also clean the existing peers.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>