plugins: Fix default trigger mode in the non-discovery path

When individual plugins (except discovery) were configured, the
default trigger mode was set as periodic. So if the plugin specified
a different mode (eg. manual), the configuration check would incorrectly
fail on account of a mode mismatch. This commit fixes the issue by
updating the trigger mode check to not specify a default mode in
scenarios when only plugins (except discovery) are configured.

Fixes: #3797

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>