Only run capabilities module in apply mode.

[seancallaway]

Running this module in check mode can cause a fatal error, as the file (normally installed by the role earlier) may not exist. Also, add a debug message indicating why this was skipped.

This resolves #72. Instead of the fatal error noted there, the following output is produced and execution continues:

TASK [blackbox-exporter : Ensure blackbox exporter binary has cap_net_raw capability] **************************************
skipping: [hostname.mydomain.com]

TASK [blackbox-exporter : Check Debug Message] *****************************************************************************
ok: [hostname.mydomain.com] => {
    "msg": "The capabilities module is skipped during check mode, as the file may not exist, causing execution to fail."
}

[paulfantom]

Thanks! 👍