Made phoebus config key optional.

Summary: For phoebus build, it is ok to not to specify phoebus options
but then users will have to specify via commands. Prev not possible
because of error check.

Brief:
For non phoebus build:
- All commnication based commands (phoebus/non phoebus) will use normal
  socket
- However there is no need to specify [Phoebus] options.

For phoebus build:
- Non phoebus commands will use normal sockets (i.e ibp_copy)
- Phoebus commands will use phoebus sockets (i.e. ibp_copy with phoebus)
- Path that will be used is as follows
  - if [Phoebus] option is specified then user can issue command with
    "auto" as path. Which will push server to use one of these paths.
  - If user specifies the path then user specified path overrides the
    config paths, and user specified path will be used for the comm.
  - If [Phoebus] option is not specified then user must specify the path
    otherwise command fails.
Added UNIS registration support

Support is optional and can be turned off either by not compining with
libunis-c or setting init_register option to 0.

Registration will help to notify unis periodically about the presence of
the ibp server. libunis-c is available with periscope.