selftests: ovpn: add prefix to helpers and shared variables

selftests: ovpn: add prefix to helpers and shared variables

Current naming for shared variables, helpers and netnamespaces is
a bit unfortunate as it doesn't come with a clean prefix.
This showed to be problematic in case of name clashes with external
scripts or in case of abrupt test termination (hanging netns' weren't
easily reconducible to ovpn).

Rename common helper entry points and all shared globals in the ovpn
selftests to ovpn_ or OVPN_ names so test scripts and wrappers use a
single explicit prefix. Also rename the temporary network namespaces
created by the tests from peerN to ovpn_peerN. This makes leaked
namespaces easier to identify.

This is a mechanical refactor only, behavior is unchanged.

Fixes: 959bc330a439 ("testing/selftests: add test tool and scripts for ovpn module")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>