kdump-lib: fix get_option_value()
    
    get_option_value() is used to get the value of $1 configured in
    /etc/kdump.conf. But when we use "get_option_value ssh", it can get the
    value of "sshkey" instead of "ssh".
    
    Fix the regexp pattern to get an exact match.
    
    Signed-off-by: WANG Chao <chaowang@redhat.com>
    Acked-by: Baoquan He <bhe@redhat.com>