1. Add sleep to avoid snapshot limitation issue https://docs.aws.amazon.com/AWSEC2/latest/APIReference/errors-overview.html#:~:text=SnapshotCreationPerVolumeRateExceeded;
2. Move InstallVelero variable out of struct of Veleroconfig as a global one since it's not for controlling any individual case;
3. Unskip migration test case on AWS pipeline, because we added a new EKS pipeline and deleted TKG AWS pipline in internal E2E test, so this restriction for TKG AWS pipline is no long existed;
4. Skip retainPV test on vSphere pipeline due to PV longtime bounding issue;
5. Fix failing get snapshot by CSI from EC2 issue, snapshot by CSI has no label of backup name.
Signed-off-by: danfengl <danfengl@vmware.com>
