Bug 1492 - java.lang.IllegalArgumentException: 'Ipv4Address
[_value=<ADDR>]' is not an IP string literal.

* changed explicit toString() call on Ipv4Address object to getValue().
* added tests for IpPrefix creation
* removed redundant fillBitMaskFromMap method

Change-Id: I38d333a126b0834c27ab0378bb7262c968b5809a
Signed-off-by: Jan Hajnar <jhajnar@cisco.com>