Do not use multiline strings for the 'stack_sshkey' option

Using multiline strings for the 'stack_sshkey' option results in the
following Puppt error:

Parameter key failed on Ssh_authorized_key[stack]: Key must not contain whitespace:

Change-Id: I41da285086af3fd90fb4887cd643d17ec52806a8
Merge "Do not use multiline strings for the 'stack_sshkey' option"