[CA-11044] & [CA-109243] Added new pool-join reasons and associated messages.

These will cover the following two cases, for XenServer v6.2.0 and later:
1) a licensed host cannot be added to an unlicensed pool;
2) an unlicensed host cannot be added to a licensed pool.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge branch 'master' of git://hg.uk.xensource.com/carbon/trunk/xenadmin