**- Why I did it**
We were building a custom version of Supervisor because I had added patches to prevent hangs and crashes if the system clock ever rolled backward. Those changes were merged into the upstream Supervisor repo as of version 3.4.0 (http://supervisord.org/changes.html#id9), therefore, we should be able to simply install the vanilla package via pip. This will also allow us to easily move to Python 3, as Python 3 support was added in version 4.0.0.
**- How I did it**
- Remove Makefiles and patches for building supervisor package from source
- Install Python 3 supervisor package version 4.2.1 in Buster base container
- Also install Python 3 version of supervisord-dependent-startup in Buster base container
- Debian package installed binary in `/usr/bin/`, but pip package installs in `/usr/local/bin/`, so rather than update all absolute paths, I changed all references to simply call `supervisord` and let the system PATH find the executable to prevent future need for changes just in case we ever need to switch back to build a Debian package, then we won't need to modify these again.
- Install Python 2 supervisor package >= 3.4.0 in Stretch and Jessie base containers
**- Why I did it**
PR https://github.com/Azure/sonic-buildimage/pull/4599 introduced two bugs in the startup of the router advertiser container:
1. References to the `wait_for_intf.sh` script were changed to `wait_for_link.sh`, but the actual script was not renamed
2. The `ipv6_found` Jinja2 variable added to the supervisor config file goes out of scope before it is read.
**- How I did it**
1. Rename the `wait_for_intf.sh` script to `wait_for_link.sh`
2. Use the Jinja2 "namespace" construct to fix the scope issue
**- How to verify it**
Ensure all processes in the radv container start properly under the correct conditions (i.e., whether or not there is at least one VLAN with an IPv6 address assigned).
Calls to sonic-cfggen is CPU expensive. This PR reduces calls to
sonic-cfggen to one call during startup when starting radv service.
singed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>
