Sentinel check to SDL CLI 'healthcheck' -command

In addition to the existing Redis master and slave healthiness
validation, add Redis sentinel healthiness validation to be done when
SDL CLI 'healthcheck' -command is ran. Validation is done by calling
Redis client's 'Sentinels' API call and parsing 'flags' field from its
output. In SDL CLI sentinel healthiness status is shown only if status
is not ok. This is done not to show too much information in CLI
interface for users.

Issue-Id: RIC-113

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I8960ec9813e06aa88217c29bac56707b38052ae7