modify ca-certificate default value
modify ca-certificate default value

modify the value from env[OS_CA_CERT] to env[OS_CACERT] to ensure the help message and codes keep consistent.

Fixes: bug 1086278

Change-Id: I0a502069cd3743f812c8145cbce73cd091e57de6

ntt-sic/python-keystoneclient,metacloud/python-keystoneclient,citrix-openstack-build/python-keystoneclient,Mercador/python-keystoneclient,metacloud/python-keystoneclient,ntt-sic/python-keystoneclient,citrix-openstack-build/python-keystoneclient,jamielennox/python-keystoneclient,jamielennox/python-keystoneclient,sdpp/python-keystoneclient,alexpilotti/python-keystoneclient,klmitch/python-keystoneclient,ging/python-keystoneclient,magic0704/python-keystoneclient,sdpp/python-keystoneclient,citrix-openstack-build/python-keystoneclient,darren-wang/ksc,ging/python-keystoneclient,ntt-sic/python-keystoneclient,Mercador/python-keystoneclient,jamielennox/python-keystoneclient,magic0704/python-keystoneclient,klmitch/python-keystoneclient,alexpilotti/python-keystoneclient,darren-wang/ksc