admin guide updates for 2.7
admin guide updates for 2.7

- Mention the file-storage-paths config file
- Add information about sharedfile and ext storage types
- Change one manpage reference to a proper link

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>
Reviewed-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>

bitemyapp/ganeti,ganeti/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,andir/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,grnet/snf-ganeti,mbakke/ganeti,yiannist/ganeti,onponomarev/ganeti,apyrgio/ganeti,yiannist/ganeti,grnet/snf-ganeti,dimara/ganeti,ganeti/ganeti,apyrgio/ganeti,bitemyapp/ganeti,onponomarev/ganeti,andir/ganeti,dimara/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,mbakke/ganeti,andir/ganeti