Bug 5390 - Adding Ipv6 L3ArbitraryBitMask feature support.

Bug 5390 - Adding Ipv6 L3ArbitraryBitMask feature support.

New yang file is created to accomodate the introduction of
ipv6 L3ArbitraryBitMask feature. It includes unit test coverage.

Sample Payload Input:
<ipv6-source-address-no-mask>2070:2070:2001:2001:2001:0471:0471:0471
</ipv6-source-address-no-mask>
<ipv6-source-arbitrary-bitmask>FFFF::0471</ipv6-source-arbitrary-bitmask>
<ipv6-destination-address-no-mask>2001:2001:2001:2001:2001:0471:0471:0471
</ipv6-destination-address-no-mask>
<ipv6-destination-arbitrary-bitmask>FFFF:FFFF::FFFF
</ipv6-destination-arbitrary-bitmask>

Change-Id: I901bd17b716fafbb44a5e0a1d7237764b14fd23c
Signed-off-by: Sai MarapaReddy <sai.marapareddy@gmail.com>
Author: Eleftherios Koxias <elkoks@intracom-telecom.com>
Author: Sai MarapaReddy <sai.marapareddy@gmail.com>