Fix lack of correct remote paths



This small changeset takes care of setting the correct remote `$PATH` *always* for all remote calls that use `check_call` fixing the lack of non-TTY configuration on some remote hosts.

It also changes how we do `disk list` to use the new wrappers and utilities, making the code easier to read and less repetitive :)

Fixes http://tracker.ceph.com/issues/6138

Example output:

    $ ceph-deploy disk list node2
    [ceph_deploy.lsb][WARNIN] lsb_release was not found - inferring OS details
    [ceph_deploy.osd][INFO  ] Distro info: CentOS 6.3 Final
    [ceph_deploy.osd][DEBUG ] Listing disks on node2...
    [node2][INFO  ] Running command: ceph-disk list
    [node2][INFO  ] /dev/sda :
    [node2][INFO  ]  /dev/sda1 other, ext4, mounted on /boot
    [node2][INFO  ]  /dev/sda2 other, LVM2_member
    [node2][INFO  ] /dev/sr0 other, unknown