dpdkevents: initialization fixes

dpdkevents: initialization fixes

This patch removes impact of keep_alive init failure on
link_status processing. Also changed default behaviour in configless
mode, where no <Plugin/> stanza is defined. Now both keep_alive
and link_status are enabled by default. On the other hand if
<Plugin/> stanza is present, both features are disabled by default
and user is responsible for specifying which one is active or not.

Change-Id: I03f7ab88e89404e03532677d00a966ddb80d1552
Signed-off-by: Krzysztof Matczak <krzysztofx.matczak@intel.com>