execvp does not require specifying the full path to the executable
(e.g., "ls" instead of "/bin/ls"), making the operation of 'lxc-attach'
consistent with 'lxc-start' and 'lxc-execute'.
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>