Utilites for manipulating base64 & PEM

Add two new utility modules

  - keystone.common.base64utils
  - keystone.common.pemutils

Add two new unit tests to exercise the above.

  - keystone.tests.test_base64utils
  - keystone.tests.test_pemutils

Functionality in these utilities will be utilized to fix bug #1233838
in a subsequent commit. It is hoped these modules will moved to Oslo
common code. Each module has a doc string describing their purpose and
functionalty.

Change-Id: I3546674dcd9eddec2c378929dd3f759f667cfd5d
Partial-Bug: #1233838
Merge "Utilites for manipulating base64 & PEM"