UserAuthServerCert: don't ask user to validate already-stored server cert
UserAuthServerCert: don't ask user to validate already-stored server cert

This way we can pre-store the server certificate and do a
noninteractive lxc remote add.  If the existing certificate
were not valid, the connection will have been refused, giving
an error of:

error: Server certificate has changed

so that this should be safe.

Signed-off-by: Serge Hallyn <3df611b026e4639bee8aef7a4beb2e39fcebb313@ubuntu.com>

achanda/lxd,smoser/lxd,sl1pm4t/lxd,stgraber/lxd,lxc/lxd,yura-pakhuchiy/lxd,jensg-st/lxd,ollie314/lxd,lxc/lxd,dustinkirkland/lxd,techtonik/lxd,atxwebs/lxd,smoser/lxd,lxc/lxd,ericsnowcurrently/lxd,pcdummy/lxd,akshaykarle/lxd,techtonik/lxd,tych0/lxd,Malizor/lxd,chrisglass/lxd,jsavikko/lxd,pcdummy/lxd,techtonik/lxd,mikemccracken/lxd,sl1pm4t/lxd,lxc/lxd,yura-pakhuchiy/lxd,sl1pm4t/lxd,atxwebs/lxd,erikmack/lxd,mjeanson/lxd,achanda/lxd,ollie314/lxd,dustinkirkland/lxd,benaryorg/lxd,smoser/lxd,achanda/lxd,atxwebs/lxd,chrisglass/lxd,ericsnowcurrently/lxd,hallyn/lxd,Malizor/lxd,benaryorg/lxd,rrva/lxd,georghopp/lxd,benaryorg/lxd,hallyn/lxd,erikmack/lxd,jensg-st/lxd,Skarlso/lxd,Skarlso/lxd,dvbportal/osx-lxd,chrisglass/lxd,yura-pakhuchiy/lxd,rcj4747/lxd,stgraber/lxd,lxc/lxd,jsavikko/lxd,basvanbeek/lxd,jumpstarter-io/lxd,rrva/lxd,stgraber/lxd,Malizor/lxd,tych0/lxd,hallyn/lxd,stgraber/lxd,ericsnowcurrently/lxd,Skarlso/lxd,beni55/lxd,jensg-st/lxd,stgraber/lxd,georghopp/lxd,akshaykarle/lxd,mikemccracken/lxd,dvbportal/osx-lxd,rcj4747/lxd,dustinkirkland/lxd,lxc/lxd,jumpstarter-io/lxd,basvanbeek/lxd,mikemccracken/lxd,pcdummy/lxd,jumpstarter-io/lxd,lxc/lxd,tych0/lxd,ollie314/lxd,hallyn/lxd,mjeanson/lxd,atxwebs/lxd,stgraber/lxd,rrva/lxd,jsavikko/lxd,beni55/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,hallyn/lxd,georghopp/lxd,basvanbeek/lxd,erikmack/lxd,akshaykarle/lxd,beni55/lxd,dvbportal/osx-lxd,mjeanson/lxd,rcj4747/lxd