Improve etcd backup filenames

[xrstf]

What does this PR do / Why do we need it:
Not all S3 backends can handle files with colons (:) in them, apparently. Also it's hard to use the files because no file extension is present and you're not even sure if it's a tar.gz of the etcd or not.

This PR addresses both points. Existing backups should be untouched, but new ones should be more compatible.

Does this PR close any issues?:
Fixes #7337

Does this PR introduce a user-facing change?:

etcd backup files are named differently (`foo-YYYY-MM-DDThh:mm:ss` to `foo-YYYY-MM-DDThhmmss.db`) to improve compatibility with different storage solutions

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [xrstf]
• pkg/controller/OWNERS [xrstf]

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[xrstf]

/retest

[xrstf]

/retest

[embik]

Q

[kubermatic-bot]

LGTM label has been added.

Git tree hash: a8f558c2a6edc7bb6ddb8bcff4c126868619d9fc

[xrstf]

/retest

[dharapvj]

/cherry-pick release/v2.19
/cherry-pick release/v2.20

[kubermatic-bot]

@dharapvj: #10143 failed to apply on top of branch "release/v2.19":

Applying: do not use colons in legacy backup file names to increase compatibility with some S3 backends
Using index info to reconstruct a base tree...
M	pkg/storeuploader/storeuploader.go
Falling back to patching base and 3-way merge...
Auto-merging pkg/storeuploader/storeuploader.go
CONFLICT (content): Merge conflict in pkg/storeuploader/storeuploader.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 do not use colons in legacy backup file names to increase compatibility with some S3 backends
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release/v2.19
/cherry-pick release/v2.20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[dharapvj]

/cherry-pick release/v2.20

[kubermatic-bot]

@dharapvj: #10143 failed to apply on top of branch "release/v2.20":

Applying: do not use colons in legacy backup file names to increase compatibility with some S3 backends
Using index info to reconstruct a base tree...
M	pkg/storeuploader/storeuploader.go
Falling back to patching base and 3-way merge...
Auto-merging pkg/storeuploader/storeuploader.go
CONFLICT (content): Merge conflict in pkg/storeuploader/storeuploader.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 do not use colons in legacy backup file names to increase compatibility with some S3 backends
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release/v2.20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.