(PUP-3357) generate implicit names for ssh_authorized_keys if needed

Generated resources from the user type share an empty resource name
if more than one unmanaged key has no comment in the authorized_keys
file (the comment is used for the namevar). This leads to duplicate
declaration errors when the user tries to purge those keys through
the user { purge_ssh_keys } parameter.

Make the provider use an internal pseudo-name in such cases. Generated
resources get the same respective pseudo-name for successful purging.
It is composed of the file name and a numeric index.
(PUP-3357) unit tests for the purging of multiple unnamed keys

Just makes sure that the user type and ssh_authorized_keys provider
generate pseudo-names for keys following the same pattern.
(PUP-3357) integration test for purging of unnamed ssh authorized keys

Add an integration test with an ssh keyfile in which two keys have no name
and make sure that both are successfully purged.