Change log level from error to warning
Change log level from error to warning

The service respawning action of ProcessMonitor is normal handling. So
the log level in external_process.py should not be error.

Closes-Bug: #1528850
Change-Id: Ie335f521b09310a577eb4fb03946dfba484eef61

wolverineav/neutron,cloudbase/neutron,bigswitch/neutron,MaximNevrov/neutron,mahak/neutron,mahak/neutron,bigswitch/neutron,openstack/neutron,huntxu/neutron,igor-toga/local-snat,huntxu/neutron,MaximNevrov/neutron,mahak/neutron,sebrandon1/neutron,wolverineav/neutron,openstack/neutron,eayunstack/neutron,noironetworks/neutron,klmitch/neutron,cloudbase/neutron,dims/neutron,dims/neutron,openstack/neutron,klmitch/neutron,igor-toga/local-snat,sebrandon1/neutron,eayunstack/neutron,noironetworks/neutron