Add check if configured directories are writable

[siegfriedweber]

Description

Check added if the following directories are writable:

• config
• data
• log
• package
• directory of the certificate file if the file does not exist
• directory of the key file if the file does not exist

Check added if the certificate and key files are readable if they exist.

Log output if a directory is not writable:

[2021-08-26T10:14:04Z ERROR stackable_agent] The directory [/] specified in the configuration option [server-cert-file] is not writable by the process.

Log output if an optional file exists but cannot be opened:

[2021-08-26T10:11:58Z ERROR stackable_agent] Could not open file [/test] which is specified in the configuration option [server-cert-file]. Permission denied (os error 13)

This change is not tested with unit tests because it is mainly about IO, so it does not make sense to try to mock the IO. Furthermore unit tests should not modify the filesystem. It is also not possible to test this change with the integration tests because they only communicate with the agent via the Kubernetes API. So they cannot manipulate the file system on the node and cannot restart the agent.

Closes #45

Review Checklist

• Code contains useful comments
• (Integration-)Test cases added (or not applicable)
• Documentation added (or not applicable)
• Changelog updated (or not applicable)

[maltesander]

LGTM.

[siegfriedweber]

I checked "(Integration-)Test cases added (or not applicable)" due to the reasons mentioned in the description.