Replace copy.deepcopy of RequestContext with copy.copy
Replace copy.deepcopy of RequestContext with copy.copy

Instance of RequestContext contains many objects and some of them like
mutexes could not be copied. Also a deepcopy of the entire
RequestContext wastes CPU time.

To avoid problems with deepcopy and avoid performance overhead this
patch changes deepcopy of RequestContext to shallow copy and makes
deepcopy of only the 'roles' member because of security issue
LP #1386932.

Closes-Bug: #1506958
Related-Bug: #1386932
Change-Id: I1e2c00e95e1c4bcd0ec7bf075458789d6fb06e99

sebrandon1/nova,klmitch/nova,cloudbase/nova,cernops/nova,cernops/nova,vmturbo/nova,rahulunair/nova,zhimin711/nova,CEG-FYP-OpenStack/scheduler,cernops/nova,gooddata/openstack-nova,bigswitch/nova,klmitch/nova,phenoxim/nova,jianghuaw/nova,vmturbo/nova,zhimin711/nova,jianghuaw/nova,mikalstill/nova,rajalokan/nova,gooddata/openstack-nova,bigswitch/nova,mahak/nova,NeCTAR-RC/nova,BeyondTheClouds/nova,rahulunair/nova,phenoxim/nova,mikalstill/nova,jianghuaw/nova,hanlind/nova,cyx1231st/nova,Juniper/nova,jianghuaw/nova,cloudbase/nova,Juniper/nova,vmturbo/nova,mahak/nova,alaski/nova,sebrandon1/nova,rahulunair/nova,CEG-FYP-OpenStack/scheduler,rajalokan/nova,rajalokan/nova,gooddata/openstack-nova,klmitch/nova,gooddata/openstack-nova,Juniper/nova,rajalokan/nova,sebrandon1/nova,mahak/nova,openstack/nova,cyx1231st/nova,Juniper/nova,mikalstill/nova,dims/nova,BeyondTheClouds/nova,NeCTAR-RC/nova,klmitch/nova,vmturbo/nova,cloudbase/nova,hanlind/nova,alaski/nova,BeyondTheClouds/nova,hanlind/nova,openstack/nova,openstack/nova,dims/nova