ceph: only use readline when in interactive mode

A mere

  import readline

line is dumping this to stdout on CentOS 6.3:

  00000000  1b 5b 3f 31 30 33 34 68  .[?1034h

That confuses non-terminals that read from stdout, so only import when we
are in the interactive mode.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>