Check VPN certificate before checking username/password

Also removes a redundant check for shill::kHostProperty

BUG=299643

Review URL: https://codereview.chromium.org/25101002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226190 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226190
Remove MOUNT_TYPE_GOOGLE_DRIVE.

The enum is no longer used. Removes it.

BUG=282104
TEST=Ran unit_tests

Review URL: https://codereview.chromium.org/25197003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226191 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226191