[PIP] Use 20.1 pip version for istanbul xtesting dockers

[PIP] Use 20.1 pip version for istanbul xtesting dockers

Using pip > 20.1 leads to docker build issues as
tags are controlled through pbr.
git history requires a target version of pbr.version.SemanticVersion(1.9.1),
but target version is pbr.version.SemanticVersion(1.0.0)

quick fix consist in setting the version 20.1 for istanbul branch
successfully tested in healthcheck
applied here to infra healthcheck and security

Issue-ID: INT-2044

Signed-off-by: morganrol <morgan.richomme@orange.com>
Change-Id: Iafe462766b2f4014a007e928520bf4fb47000feb