common: CEPH_ARGS should trim whitespaces

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>