CLOUDSTACK-4770: In MacAddress skip macAddress when parsed value is 0x00

In MacAddress class, we start by settig macAddress String as null and go through
the output of ifconfig -a and pick the one string that is a valid mac address
but is not 0x00 and 0xff. With each loop we set the macAddress to null so that
it does not pick the last one if everything fails.

Tested on Ubuntu where I had an interface called cloud0 whose mac id was 0x00
and it was skipped to get the next one:

$ java -classpath <path-to-cloud-utils.jar> com.cloud.utils.net.MacAddress
addr in integer is 5071953436
addr in bytes is  0 1 2e 4f de 1c
addr in char is 00:01:2e:4f:de:1c

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
(cherry picked from commit 3b5aa42c6d87fb7e2573146efb8c7d2d0a4692b3)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
NetUtils: Check for NPE in getDefaultHostIp method when processing nic/mac

On hosts or containers where they don't have valid mac address on nic resulting
in null, NetUtils.getNetworkParam can throw NPE.

This was a case found on TravisCI where OpenVZ containers are used. This method
(getDefaultHostIp) is used at several other places within the ACS codebase to
get the host IP and if null is caught we fallback to localhost or 127.0.0.1, so
we therefore set info to null before trying to process network param and if we
fail we return null and expect other layers to use localhost.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>
(cherry picked from commit 7ada4ad50b683268f4031ff05e9d19d15d2c35f1)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Conflicts:
	utils/src/com/cloud/utils/net/NetUtils.java