(PUP-1955) Use symbols not booleans in purge_ssh_keys

Unfortunately, defaultto false is somewhat broken: a parameter default of
false is cast to nil (PUP-1955). This patch alters the boolean
representation of the User Type parameter purge_ssh_keys to use Symbols
instead of booleans as deafaultto :false works correctly.

Also added some logic to convert boolean values (true) and string values
("true") to symbols during munging. Both of these cases can occur depending
on whether a quoted string or bare word is used to define a parameter in the
Puppet manifest.