scylla_setup: match '-p' option of lsblk with strict pattern
scylla_setup: match '-p' option of lsblk with strict pattern

On Ubuntu 14.04, the lsblk doesn't have '-p' option, but
`scylla_setup` try to get block list by `lsblk -pnr` and
trigger error.

Current simple pattern will match all help content, it might
match wrong options.
  scylla-test@amos-ubuntu-1404:~$ lsblk --help | grep -e -p
   -m, --perms          output info about permissions
   -P, --pairs          use key="value" output format

Let's use strict pattern to only match option at the head. Example:
  scylla-test@amos-ubuntu-1404:~$ lsblk --help | grep -e '^\s*-D'
   -D, --discard        print discard capabilities

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>
Message-Id: <4f0f318353a43664e27da8a66855f5831457f061.1489712867.git.eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

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