topotests: structural issues in bgp_local_as_dotplus_private_remove

topotests: structural issues in bgp_local_as_dotplus_private_remove

This test has several issues:
A) The convergence function is spamming the show neighbor command until success,
if the neighbor never comes up the test will never finish. This adds unnecessary
load to an already loaded test system.  Use run_and_expect to properly wait for
the neighbor relationship to come up.
B) The convergence function should not sleep for 1 second *After* the neighbor
is established
C) The _bgp_as_path() function fails if the prefix has not been received yet.
This looking for the prefix data should be within a run_and_expect() functionality.
Else a loaded test system will fail in this function because while we may be in
an established state, prefixes might not yet have been exchanged and there is no
point in failing the test without giving the system some time to actually converge.

Fix those points, similarly to what has been fixed in
bgp_local_as_private_remove test.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>