Agent should check on startup if important directories are readable/writable

[lfrancke]

When the data directory is not writeable the Agent will exit with a weird panic from the bowels of Krustlet because the plugins dir cannot be created. To avoid this we should check if the data directory exists and is writeable so we can provide a nicer error.

The same might be nice for the log dir and maybe others? Not sure where it makes sense.

[siegfriedweber]

The agent should check 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

The agent should check if the certificate and key files are readable if they exist.

If a check fails then an error message should be logged. The agent should not terminate because a failed check does not necessarily mean that an error will occur, e.g. if the log directory is not used.

As a next step, missing directories should be created (see #105).