unable to terminate netconf session due to process ID is truncated to 4 digits

In Netconf MDSAL test suites, the aborting simulation test will try to terminate sshpass process to simulate the abort netconf session.

the current method in robot test to obtain the process ID of sshpass is 

${pid}=    SSHLibrary.Execute_Command    ps -A | grep sshpass | cut -b 1-6

It works if process ID is 4 digit or less, but failed to terminate sshpass if process ID is 5 digit

$ ps -A | grep sshpass
  13090 pts/2    00:00:00 sshpass
$ ps -A | grep sshpass | cut -b 1-6
  1309