[SPARK-8018] [MLLIB] KMeans should accept initial cluster centers as param

 This allows Kmeans to be initialized using an existing set of cluster centers provided as  a KMeansModel object. This mode of initialization performs a single run.

Author: FlytxtRnD <meethu.mathew@flytxt.com>

Closes #6737 from FlytxtRnD/Kmeans-8018 and squashes the following commits:

94b56df [FlytxtRnD] style correction
ef95ee2 [FlytxtRnD] style correction
c446c58 [FlytxtRnD] documentation and numRuns warning change
06d13ef [FlytxtRnD] numRuns corrected
d12336e [FlytxtRnD] numRuns variable modifications
07f8554 [FlytxtRnD] remove setRuns from setIntialModel
e721dfe [FlytxtRnD] Merge remote-tracking branch 'upstream/master' into Kmeans-8018
242ead1 [FlytxtRnD] corrected == to === in assert
714acb5 [FlytxtRnD] added numRuns
60c8ce2 [FlytxtRnD] ignore runs parameter and initialModel test suite changed
582e6d9 [FlytxtRnD] Merge remote-tracking branch 'upstream/master' into Kmeans-8018
3f5fc8e [FlytxtRnD] test case modified and one runs condition added
cd5dc5c [FlytxtRnD] Merge remote-tracking branch 'upstream/master' into Kmeans-8018
16f1b53 [FlytxtRnD] Merge branch 'Kmeans-8018', remote-tracking branch 'upstream/master' into Kmeans-8018
e9c35d7 [FlytxtRnD] Remove getInitialModel and match cluster count criteria
6959861 [FlytxtRnD] Accept initial cluster centers in KMeans