Added 'config core' and 'show cores' CLI commands

[rajendra-dendukuri]

- What I did
Added wrapper CLI commands to see core files information. Also added CLI command to enable/disable
corefile generation.

- How I did it
Added new commands
config core
show cores

- How to verify it

show cores list
config core enable
config core disable

- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

root@sonic:/home/admin# show cores
Usage: show cores [OPTIONS] COMMAND [ARGS]...

Show core dump events encountered

Options:
-?, -h, --help Show this message and exit.

Commands:
config Show coredump configuration
info Show information about one or more coredumps
list List available coredumps
root@sonic:/home/admin# show cores list
TIME PID UID GID SIG COREFILE EXE
Sat 2016-12-31 22:46:22 UTC 4933 0 0 6 present /usr/bin/vlanmgrd
root@sonic:/home/admin# show cores info
PID: 4933 (vlanmgrd)
UID: 0 (root)
GID: 0 (root)
Signal: 6 (ABRT)
Timestamp: Sat 2016-12-31 22:46:22 UTC (6min ago)
Command Line: /usr/bin/vlanmgrd
Executable: /usr/bin/vlanmgrd
Control Group: /docker/38e40a3313bd46ff4e282f897cdc25502a2b15febe9839715f92b035b6f2cffe
Slice: -.slice
Boot ID: 3f776b83fa704d089bd81e0b108c2a09
Machine ID: a0c0523aae8c4c21a8d9fa12e18d85b0
Hostname: sonic
Storage: /var/lib/systemd/coredump/core.vlanmgrd.0.3f776b83fa704d089bd81e0b108c2a09.4933.1483224382000000000000.lz4
Message: Process 4933 (vlanmgrd) of user 0 dumped core.

            Stack trace of thread 90:
            #0  0x00007f69d80cb303 epoll_wait (libc.so.6)
            #1  0x00007f69d9076ef8 n/a (/usr/lib/x86_64-linux-gnu/libswsscommon.so.0.0.0)

root@sonic:/home/admin#

-->

[rajendra-dendukuri]

Aslo refer to
sonic-net/sonic-buildimage#3499
sonic-net/SONiC#468