tests: Add test suite shared identity management fixture

This is one of the two commits for issue #1683.
This one provides an identity management fixture which will be used to replace the existing
global IdentityFixture in the second commit.

Change-Id: I2eecaa2ee73661db328760ac957474791ff59112
Refs: #1683
tests: Manage identity/certificate creation in test cases through IdentityManagementFixture.

This is the second one of the two commits for issue #1683.
For test cases which create new identities/certificates, the temporary identities/certificates
are managed by IdentityManagementFixture.

Change-Id: I5b42348bd74091a28042aea399484fb4ed1b58f4
Refs: #1683