common: CEPH_ARGS should trim whitespaces

CEPH_ARGS when parsed by env_to_vec did not trim trailing and leading
whitespaces: they would unexpectedly be parsed as empty arguments and
lead to confusing errors such as :

    CEPH_ARGS=' --id 3' ceph-conf --lookup mon-data
    unable to parse option: ''

The parsing code is replaced with str_vec(). It also resolves the
problem of the hard limit to 1000 characters imposed by the static
buffer that would silently truncate any CEPH_ARGS content.

Signed-off-by: Loic Dachary <loic@dachary.org>
ceph-conf: display arguments when an option cannot be parsed

   unable to parse option: ''

Is difficult to figure out. It is much better if shown in the context in
which it was found:

   ceph-conf --help ''
   unable to parse option: ''
   args: '--help' ''

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1031 from dachary/wip-parse-args

CEPH_ARGS should trim whitespaces

Reviewed-by: Sage Weil <sage@inktank.com>