drop CAP_SYSLOG capability

Kernel capabilities for privileged syslog operations are currently splitted into
CAP_SYS_ADMIN and CAP_SYSLOG since the following commit:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ce6ada35bdf710d16582cc4869c26722547e6f11

This patch drops CAP_SYSLOG to prevent containers from messing with
host's syslog (e.g. `dmesg -c` clears up host's printk ring buffer).

Closes #5491

Docker-DCO-1.1-Signed-off-by: Eiichi Tsukata <devel@etsukata.com> (github: Etsukata)