allow to set number of bits in manager._create_subnet

Change-Id: I2a354bde18122e5c547a7ef5c1c5075a9e147ed8
Yaroslav's special

Change-Id: I640e98920498543d14fe0cba75bb24eabfecf6d7
changes after review

Change-Id: Ia3446afb417f1955fe4daf068c14f5b63379ae01
Merge pull request #12 from kshileev/ipv6

general ipv6 infra with optional net_bits and Yaroslav's special