LU-16822 tests: Force IPv6 testing in mixed environment

LU-16822 tests: Force IPv6 testing in mixed environment

When an interface has both IPv4 and v6 addresses LNet will, by
default, configure the NI using the v4 address. The '--large' option
to lnetctl lnet configure tells LNet to configure the NI using the
v6 address instead. This patch adds a test-framework environment
variable that, when set, passes the --large option to lnet configure.
This allows us to force testing of IPv6 when running in a mixed v4/v6
environment.

This patch implements ip_is_v6() which is needed by some of the
router tests when using IPv6 NIDs.

Some test cases are added to the except list:
 - 230 requires lctl conn_list to be updated to work with large nids.
 - 303, and 500 have been found to trip LU-17460 bug.

Change-Id: I8934a87bfd836779b167df39c5d09d97ff78debf
Test-Parameters: trivial
Signed-off-by: Srikanth Ramamurthy <srramamu@microsoft.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55422
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>