rbd-ggate: filter out ceph options from args

rbd-ggate: filter out ceph options from args

we can just filter out ceph options from command line args instead of
parsing config in env variables, conf files and argvs.

the config-parsing steps were modeled after the ones in rbd-nbd.cc, but
the whole purpose of reading configurations from env/conf-file is to get
the "rbd_default_pool" for setting cfg->poolname. apparently, this does
not apply to rbd-ggate, where global_init() will take care of all the
trivial work to populate the settings. moreover, `config` is not
read after being initialized in main.cc. so drop it.

Signed-off-by: Mykola Golub <mgolub@suse.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>