postgresql_membership: add the exact state value to be able to specify a list of only groups a user must be a member of

[Andersson007]

SUMMARY

Fixes #277

ISSUE TYPE

• Feature Pull Request

[Andersson007]

@hunleyd @Vanav @jchancojr how about (at least) the interface implementation?
when we pass:

postgresql_membership:
  groups: []
  state: exact
  user: blah

it'll revoke all groups from a user (i.e. all not listed under groups).

Resp if we pass

postgresql_membership:
  groups:
    - group1
    - group2
  state: exact
  user: blah

group1 and group2 will be granted to blah if he's already not in them and all other groups will be revoked.

i'll be polishing the implementation and the question now is about the interface

[Vanav]

Looks good, thank you!

[hunleyd]

LGTM

[Andersson007]

Ready for review folks.
This time I'll ask to take a look at the code changes please

[Andersson007]

@hunleyd @Vanav thanks for reviewing!