MB-7720: deferred creation of the sample and default buckets

.. to the very end of Create Cluster wizard
eliminated flicker on default buckets page
replaced "loading buckets" dialog with a spinner
removed "Installed Samples" section from the sample buckets page
removed setupDefaultBucket REST API

Change-Id: I561ac0d76bc220be682ea55a97f699b410c85dc7
[alk@tut.by: fixed minor formatting issue]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/27314
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>