master: Modify algorithm of creating new chunks to respect labels

This commit changes the algorithm which chooses chunkservers to place
a new chunk. Now it makes use of custom goals and media labels.

Te algorithm is changed in a way that it no longer maintains 'carry'
between runs, which means that for each new chunk each chunkserver is
chosen completely randomly (with respect to weights equal its total disk
space though). The previous implementation was trying to distribute
chunks more evenly, but making it random shouldn't influence the
observed behavior of the system.

Change-Id: I335e6b116fa9c1678665781cf52f675533c316d0
tests: Add a test test_creating_new_chunks_with_labels

This test verifies if new chunks are created with respect to
labels of chunkservers.

Change-Id: I2f847f9b113027c7feca06b2ab95ea74ec81d8ef