Description of problem:

Carry Downstream: Override kopia default hash, encryption, splitter algo via env var. 

As seen in: https://github.com/openshift/velero/pull/329/files

As a user, developer, performance engineer... I would like to be able to set the hash, encryption, splitter algorithms to achieve better performance or to compare the performance with kopia.

More information regarding the various options and benchmarking can be seen here:

https://hackmd.io/Jc7oHHwqQxKD6M1VSqCrrw

Reminder:

squash https://github.com/openshift/velero/pull/334 and spampatt@redhat.com  PR https://github.com/openshift/velero/pull/337  into one commit to make it easier to carry.

&& cp over to konveyor-dev.