topotests: add support for frr.conf as a unified config

topotests: add support for frr.conf as a unified config

This PR adds support for configuring topotest routers using a single file.
  instead of:
```
        router.load_config(
	    TopoRouter.RD_ZEBRA, os.path.join(CWD, "{}/zebra.conf".format(rname))
	    )
	router.load_config(
	    TopoRouter.RD_OSPF, os.path.join(CWD, "{}/ospfd.conf".format(rname))
	    )
	router.load_config(
	    TopoRouter.RD_BGP, os.path.join(CWD, "{}/bgpd.conf".format(rname))
	    )
```
  you can now do:
```
        router.load_frr_config(
	    os.path.join(CWD, "{}/frr.conf".format(rname)),
	    [TopoRouter.RD_ZEBRA, TopoRouter.RD_OSPF, TopoRouter.RD_BGP]
	    )
```
or just:

```
        router.load_frr_config(os.path.join(CWD, "{}/frr.conf".format(rname)))
```
In this latter case, the daemons list will be inferred from frr.conf file.

Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>