Fix ENOENT error from backup_pg_compress

[agrare]

Dir.mktmpdir fails if the base directory doesn't exist with ENOENT so
first make sure that the base path exists.

https://travis-ci.org/ManageIQ/manageiq/jobs/333519975#L1334-L1350

Failures:
  1) EvmDatabaseOps#backup remotely without a remote file name
     Failure/Error: backup = PostgresAdmin.backup(db_opts)
     Errno::ENOENT:
       No such file or directory @ dir_s_mkdir - /home/travis/build/ManageIQ/manageiq/tmp/db_backup/vmdb_backup20180125-7288-cziha5
     # ./lib/evm_database_ops.rb:64:in `backup'
     # ./spec/lib/evm_database_ops_spec.rb:45:in `block (3 levels) in <top (required)>'
  2) EvmDatabaseOps#backup remotely
     Failure/Error: backup = PostgresAdmin.backup(db_opts)
     Errno::ENOENT:
       No such file or directory @ dir_s_mkdir - /home/travis/build/ManageIQ/manageiq/tmp/db_backup/vmdb_backup20180125-7288-1wok2su
     # ./lib/evm_database_ops.rb:64:in `backup'

[agrare]

cc @kbrock

[miq-bot]

Checked commit agrare@b391b10 with ruby 2.3.3, rubocop 0.52.0, haml-lint 0.20.0, and yamllint 1.10.0
1 file checked, 0 offenses detected
Everything looks fine. 🏆

[agrare]

Not sure if there's a better place to initialize this directory but this fixes the test

[bdunne]

Based on changes in #331

[agrare]

Thanks @bdunne 👍

[carbonin]

Thanks for catching this @agrare

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 22145df40925eea3ef7fbe5837f278ed6a1c01b6
Author: Brandon Dunne <[email protected]>
Date:   Thu Jan 25 20:45:27 2018 -0500

    Merge pull request #332 from agrare/fix_pg_backup_error_no_such_file_or_dir
    
    Fix ENOENT error from backup_pg_compress
    (cherry picked from commit cd8fcb98fa8bed74565ad4e95c3cc9ca44b69241)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1539060