Ignore slow log configuration on shard creation

In case of a misconfigured slow search/index configuration (unparseable
TimeValue) an exception is thrown.

This is not a problem when creating a shard of an index, as an exception
is returned and all is good. However, this is a huge problem, when
starting up a node, as the shard creation is repeated endlessly.

This patch changes the behaviour to go on as usual and just disable the
slowlog, as an improper configuration of logging should not affect the
allocation behaviour.

Closes #2730