Fix upgrade with network problems

After interruption of upgrade process with some network problems there should be possibility to finish the upgrade successfully.

Steps to reproduce:

1. Set-up previous version of PMM and start upgrade;

2. Then simulate some network problems and verify upgrade process stops with notification.

3. Verify upgrade process can be started after problems fixed and can be finished successfully

Expected result:

Error message after several  attempts on the backend
and roll back all new packages ... so the user will 
1 . "Get error message "
2 . Left in the state he started upgrade (so on older packages )

Actual result:

User can see some not friendly error but can not be returned to the state before the upgrade.

Suggested implementation:

• Use version of pmm-managed package instead of pmm-update's for checking if there a newer version

• replace pmm-update with pmm-managed in following places

  • https://github.com/percona/pmm-update/blob/bb7c23ed043393d099a7857aef6bf578b101d06a/main.go#L36

  • https://github.com/percona/pmm-update/blob/bb7c23ed043393d099a7857aef6bf578b101d06a/main.go#L47

Note:

There might be some changes in upgrade testing process. We probably won't need to update pmm-update before each release. But we might have to update pmm-managed instead of pmm-update.

We still get changelog from pmm-update package. Unless we want to implement update via SaaS service