Allow shard settle timeout to be tuned

Allow shard settle timeout to be tuned

When we are starting the datastore, we wait up to a fixed number of
election timeout intervals (3) for shards to finish electing a leader.

This is not always enough, especially if recovery takes a long time,
hence introduce shardLeaderElectionTimeoutMultiplier to make this
tunable.

JIRA: CONTROLLER-1914
Change-Id: Iba1d116d0248fc6046aeeae3ec30ecac50f373c9
Signed-off-by: Tibor Král <tibor.kral@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>