Fix mongodb database existance check

The mongodb_database provider is not properly detecting the presence of the last database in a list of databases with mongodb enterprise (specifically tested on 2.6.0). I do not know if this impacts other versions of mongodb.

getDBNames() is returning a newline (\n) and so fails the exists test as demonstrated in the following ruby code example:

#!/usr/bin/ruby
p %x(/usr/bin/mongo --quiet --eval 'db.getMongo().getDBNames()').split(",")
p %x(/usr/bin/mongo --quiet --eval 'db.getMongo().getDBNames()').split(",").include?('mydb2')

# /tmp/test.rb
["mydb1", "admin", "local", "mydb2\n"]
false

A chomp should be added to strip any newlines as demonstrated in the following ruby code example:

#!/usr/bin/ruby
p %x(/usr/bin/mongo --quiet --eval 'db.getMongo().getDBNames()').chomp.split(",")
p %x(/usr/bin/mongo --quiet --eval 'db.getMongo().getDBNames()').chomp.split(",").include?('mydb2')

# /tmp/test2.rb
["mydb1", "admin", "local", "mydb2"]
true