We currently limit backups to the same AZ of the volume while allowing
volume restoration to any AZ.
When having multiple AZs it is ideal to store your backups in a
different AZ than the one where the source volumes are stored.
This patch adds microversion 3.51 where we allow requesting the AZ where
we want to store backups which will allow us to have as many backup AZs
as volume AZs and do cross AZ backups for increased security, this
feature also supports having an additional AZ just for backups or the
less desirable solution where you store all your backups in a single AZ
shared with some of the volumes.
Change-Id: I595932276088d25abd464025c99dce33a2cc502b
5feaf74ccf