[broadcom]: Fix BRCM Syncd Error:syncd#/supervisord: syncd sh: 1: ethtool: not found (sonic-net#6615)

Starting with BRCM SAI 4.3.1.5 we see the following :ethtool not fount" error in syslog during boot up:
```
Jan 27 07:36:14.712472 str-s6100-acs-1 INFO syncd#/supervisord: syncd sh: 1:
Jan 27 07:36:14.712844 str-s6100-acs-1 INFO syncd#/supervisord: syncd ethtool: not found
Jan 27 07:36:14.713228 str-s6100-acs-1 INFO syncd#/supervisord: syncd #15
Jan 27 07:36:14.713840 str-s6100-acs-1 INFO syncd#syncd: [0] SAI_API_HOSTIF:_brcm_sai_hostif_speed_set:11894 cmd ethtool -s Ethernet39 speed 40000 rc:32512
Jan 27 07:36:14.717204 str-s6100-acs-1 NOTICE swss#orchagent: :- setHostIntfsOperStatus: Set operation status DOWN to host interface Ethernet39
Jan 27 07:36:14.717204 str-s6100-acs-1 NOTICE swss#orchagent: :- initPort: Initialized port Ethernet39
Jan 27 07:36:14.717204 str-s6100-acs-1 NOTICE swss#orchagent: :- initializePort: Initializing port alias:Ethernet36 pid:1000000000040
Jan 27 07:36:14.726793 str-s6100-acs-1 NOTICE swss#portsyncd: :- onMsg: nlmsg type:16 key:Ethernet36 admin:0 oper:0 addr:4c:76:25:f5:48:80 ifindex:75 master:0
Jan 27 07:36:14.727967 str-s6100-acs-1 NOTICE swss#portsyncd: :- onMsg: Publish Ethernet36(ok) to state db
Jan 27 07:36:14.729331 str-s6100-acs-1 NOTICE swss#orchagent: :- addHostIntfs: Create host interface for port Ethernet36
Jan 27 07:36:14.752398 str-s6100-acs-1 INFO syncd#/supervisord: syncd sh: 1: ethtool: not found#015
Jan 27 07:36:14.752689 str-s6100-acs-1 INFO syncd#syncd: [0] SAI_API_HOSTIF:_brcm_sai_hostif_speed_set:11894 cmd ethtool -s Ethernet36 speed 40000 rc:32512
Jan 27 07:36:14.756050 str-s6100-acs-1 NOTICE swss#orchagent: :- setHostIntfsOperStatus: Set operation status DOWN to host interface Ethernet36
Jan 27 07:36:14.757585 str-s6100-acs-1 NOTICE swss#orchagent: :- initPort: Initialized port Ethernet36
```
It seems that starting with BRCM SAI 4.2.1.5 syncd is using ethtool to set the host interface speed and since this ethtool was not part of the syncd Docker, we observe these "ethtool not found" issue.

Author: gechiang

platform/broadcom/docker-syncd-brcm/Dockerfile.j2

@@ -21,6 +21,9 @@ debs/
 ## TODO: add kmod into Depends
 RUN apt-get install -yf kmod
 
+## BRCM uses ethtool to set host interface speed
+RUN apt-get install -y ethtool
+
 COPY ["files/dsserve", "files/bcmcmd", "start.sh", "start_led.sh", "bcmsh", "/usr/bin/"]
 RUN chmod +x /usr/bin/dsserve /usr/bin/bcmcmd