[release/v2.15] fix CA's not being properly mounted on CentOS

[xrstf]

What this PR does / why we need it:
This is a temporary fix for 2.15 to allow customers to upgrade, as 2.16+ ships with the new CA handling (#6538).
The root cause is Fedora/CentOS storing their trust anchors as

[root@xrstf-ssl-mount-test certs]# pwd && ls -lah
/etc/ssl/certs
total 8.0K
drwxr-xr-x. 2 root root 4.0K Feb 14 05:41 .
drwxr-xr-x. 5 root root 4.0K Feb 14 05:41 ..
lrwxrwxrwx. 1 root root   49 Aug 12  2020 ca-bundle.crt -> /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem
lrwxrwxrwx. 1 root root   55 Aug 12  2020 ca-bundle.trust.crt -> /etc/pki/ca-trust/extracted/openssl/ca-bundle.trust.crt

Which issue(s) this PR fixes:
Fixes #6469

Does this PR introduce a user-facing change?:

Fix CA certificates not being mounted on Seed clusters using CentOS/Fedora.

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xrstf

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[kubermatic-bot]

LGTM label has been added.

Git tree hash: 02074c3f7981e6cf0f4a82eda73644979e6f8f0d

[xrstf]

/retest

[xrstf]

/retest

[xmudrii]

/cherrypick release/v2.16

[kubermatic-bot]

@xmudrii: new pull request created: #7056

In response to this:

/cherrypick release/v2.16

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